getFab should return fab (#615)

android/app/src/main/java/com/reactnativenavigation/params/ScreenParams.java

     }
 
     public FabParams getFab() {
-        return hasTopTabs() ? topTabParams.get(0).fabParams : null;
+        return hasTopTabs() ? topTabParams.get(0).fabParams : fabParams;
     }
 }