Bug 827860 - Part 1: STK get stuck when text string is null, r=vicamo,yhuang

dom/system/gonk/ril_worker.js

@@ -7693,7 +7693,7 @@ let StkProactiveCmdHelper = {
   retrieveTextString: function retrieveTextString(length) {
     if (!length) {
       // null string.
-      return null;
+      return {textString: null};
     }
 
     let text = {