Merge back button only if it's defined explicitly

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

         BackButton result = new BackButton();
         if (json == null) return result;
 
+        result.hasValue = true;
         result.visible = BoolParser.parse(json, "visible");
         if (json.has("icon")) result.icon = TextParser.parse(json.optJSONObject("icon"), "uri");
         result.id = json.optString("id", Constants.BACK_BUTTON_ID);
         return result;
     }
 
-    public BackButton() {
+    BackButton() {
         id = Constants.BACK_BUTTON_ID;
     }
 
     public Bool visible = new NullBool();
+    private boolean hasValue;
+
+    public boolean hasValue() {
+        return hasValue;
+    }
 
     public void mergeWith(BackButton other) {
         if (other.icon.hasValue()) icon = other.icon;

lib/android/app/src/main/java/com/reactnativenavigation/presentation/StackOptionsPresenter.java

     private void mergeButtons(TopBarButtons buttons) {
         if (buttons.left != null) topBar.setLeftButtons(buttons.left);
         if (buttons.right != null) topBar.setRightButtons(buttons.right);
-        if (buttons.back != null) topBar.setBackButton(buttons.back);
+        if (buttons.back.hasValue()) topBar.setBackButton(buttons.back);
     }
 
     private void mergeTopBarOptions(TopBarOptions options, AnimationsOptions animationsOptions, Component component) {