CB-12218: (android) Fix consistency of null result message

Fix problem that JavaScript receives ""(empty string) instead of null
if plugin sends null result from new thread.

framework/src/org/apache/cordova/NativeToJsMessageQueue.java

@@ -511,6 +511,9 @@ public class NativeToJsMessageQueue {
                             .append(pluginResult.getMessage())
                             .append("')");
                     break;
+                case PluginResult.MESSAGE_TYPE_NULL:
+                    sb.append("null");
+                    break;
                 default:
                     sb.append(pluginResult.getMessage());
             }