fix json parse error (#327)

Author: changxu0306

aws-android-sdk-appsync/src/main/java/com/amazonaws/mobileconnectors/appsync/AppSyncCustomNetworkInvoker.java

@@ -240,7 +240,7 @@ public void onResponse(@Nonnull Call call, @Nonnull Response response) throws IO
 
                                                    if (persistentMutationsCallback != null) {
                                                        persistentMutationsCallback.onResponse(new PersistentMutationsResponse(
-                                                               jsonObject.getJSONObject("data"),
+                                                               jsonObject.optJSONObject("data"),
                                                                jsonObject.optJSONArray("errors"),
                                                                persistentOfflineMutationObject.responseClassName,
                                                                persistentOfflineMutationObject.recordIdentifier));