Changed return value to null for getUpdateMetadata instead of empty string

In accordance with docs and #862

Author: max-mironov

android/app/src/main/java/com/microsoft/codepush/react/CodePushNativeModule.java

@@ -321,7 +321,7 @@ protected Void doInBackground(Void... params) {
                 JSONObject currentPackage = mUpdateManager.getCurrentPackage();
 
                 if (currentPackage == null) {
-                    promise.resolve("");
+                    promise.resolve(null);
                     return null;
                 }
 
@@ -335,14 +335,14 @@ protected Void doInBackground(Void... params) {
                 if (updateState == CodePushUpdateState.PENDING.getValue() && !currentUpdateIsPending) {
                     // The caller wanted a pending update
                     // but there isn't currently one.
-                    promise.resolve("");
+                    promise.resolve(null);
                 } else if (updateState == CodePushUpdateState.RUNNING.getValue() && currentUpdateIsPending) {
                     // The caller wants the running update, but the current
                     // one is pending, so we need to grab the previous.
                     JSONObject previousPackage = mUpdateManager.getPreviousPackage();
 
                     if (previousPackage == null) {
-                        promise.resolve("");
+                        promise.resolve(null);
                         return null;
                     }