Don't parse back button is json is empty

lib/android/app/src/main/java/com/reactnativenavigation/parse/BackButton.java

 public class BackButton extends Button {
     public static BackButton parse(JSONObject json) {
         BackButton result = new BackButton();
-        if (json == null) return result;
+        if (json == null || json.toString().equals("{}")) return result;
 
         result.hasValue = true;
         result.visible = BoolParser.parse(json, "visible");