Error parsing data org.json.JSONException , already try other post

I know there are several posts with the same error, but I've proven solutions and still have the same error

Log.txt:

03-18 18:32:33.082: D/gralloc_goldfish(974): Emulator without GPU emulation detected.
03-18 18:33:21.706: E/JSON Parser(974): Error parsing data org.json.JSONException: End of input at character 0 of 
03-18 18:33:21.706: W/dalvikvm(974): threadid=11: thread exiting with uncaught exception (group=0x40a71930)
03-18 18:33:21.726: E/AndroidRuntime(974): FATAL EXCEPTION: AsyncTask #1
03-18 18:33:21.726: E/AndroidRuntime(974): java.lang.RuntimeException: An error occured while executing doInBackground()
03-18 18:33:21.726: E/AndroidRuntime(974):  at android.os.AsyncTask$3.done(AsyncTask.java:299)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.util.concurrent.FutureTask.setException(FutureTask.java:219)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.util.concurrent.FutureTask.run(FutureTask.java:239)
03-18 18:33:21.726: E/AndroidRuntime(974):  at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.lang.Thread.run(Thread.java:856)
03-18 18:33:21.726: E/AndroidRuntime(974): Caused by: java.lang.NullPointerException
03-18 18:33:21.726: E/AndroidRuntime(974):  at com.example.cambio_moneda.LoadAllProducts.doInBackground(LoadAllProducts.java:25)
03-18 18:33:21.726: E/AndroidRuntime(974):  at com.example.cambio_moneda.LoadAllProducts.doInBackground(LoadAllProducts.java:1)
03-18 18:33:21.726: E/AndroidRuntime(974):  at android.os.AsyncTask$2.call(AsyncTask.java:287)
03-18 18:33:21.726: E/AndroidRuntime(974):  at java.util.concurrent.FutureTask.run(FutureTask.java:234)
03-18 18:33:21.726: E/AndroidRuntime(974):  ... 4 more
03-18 18:33:21.786: D/dalvikvm(974): GC_CONCURRENT freed 149K, 10% free 2622K/2896K, paused 4ms+5ms, total 43ms

MainActivity.java

package com.example.cambio_moneda;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends Activity {
    JSONParser jParser = new JSONParser();
    private TextView tvCambio;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tvCambio = (TextView) findViewById(R.id.tvCambio);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    public void consultarCambio(View v){
        new LoadAllProducts().execute();
    }
}

LoadAllProducts.java

package com.example.cambio_moneda;

import java.util.ArrayList;
import java.util.List;
import org.apache.http.NameValuePair;
import org.json.JSONObject;
import android.os.AsyncTask;
import android.util.Log;

class LoadAllProducts  extends AsyncTask<String, String, String>{

    JSONParser jsonParser = new JSONParser();
    private static String url_all_products = "http://10.10.1.40/cordova/Servidor/cambio_moneda_app/index.php";


    @Override
    protected void onPreExecute(){
    }

    @Override
    protected String doInBackground(String... params) {
        List<NameValuePair> params1 = new ArrayList<NameValuePair>();

        JSONObject json = jsonParser.makeHttpRequest(url_all_products,"POST",params1);
        Log.d("All Products: ", json.toString());
        return null;
    }
}

JSONParser.java

package com.example.cambio_moneda;

import java.io.InputStream;
import java.util.List;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.utils.URLEncodedUtils;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONException;
import org.json.JSONObject;

import android.util.Log;

public class JSONParser {
    static InputStream is = null;
    static JSONObject JObj = null;
    static String json = "";

    // Constructor
    public JSONParser() {

    }

    public JSONObject makeHttpRequest(String url, String method,
            List<NameValuePair> params) {
        try {
            if (method == "POST") {
                DefaultHttpClient httpClient = new DefaultHttpClient();
                HttpPost httpPost = new HttpPost(url);
                httpPost.setEntity(new UrlEncodedFormEntity(params));

                HttpResponse httpResponse = httpClient.execute(httpPost);
                HttpEntity httpEntity = httpResponse.getEntity();
                is = httpEntity.getContent();
            } else if (method == "GET") {
                // request method is GET
                DefaultHttpClient httpClient = new DefaultHttpClient();
                String paramString = URLEncodedUtils.format(params, "utf-8");
                url += "?" + paramString;
                HttpGet httpGet = new HttpGet(url);

                HttpResponse httpResponse = httpClient.execute(httpGet);
                HttpEntity httpEntity = httpResponse.getEntity();
                is = httpEntity.getContent();
            }

        } catch (Exception e) {
            e.printStackTrace();
        }

        try{
            JObj = new JSONObject(json);
        } catch (JSONException e){
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }

        return JObj;
    }
}

index.php

<?php

require_once 'include/app_android/DB_Functions.php';
$db = new DB_Functions();

$response = array();

$result = $db->get_cambio();
if (mysql_num_rows($result) != 0) {
    while($reg = mysql_fetch_array($result)){
        $fecha = date_create($reg['fecha']);
        $response['fecha'] = date_format($fecha, 'd-m-Y')." ".date_format($fecha, 'H:i:s');
        $response['cambio'] = $reg['cambio'];
    }
    echo json_encode($response);
}
?>

I am new to all this, but I really do not watch what may be the mistake, as I said above, I followed the other post, and I don't find the error

Thank you all.

PD: In some post gave me negative points, please if I give them, leave a comment to know that I am wrong