Set default fontFamily

fixes #2038

android/app/src/main/java/com/reactnativenavigation/params/parsers/StyleParamsParser.java

         result.titleBarHideOnScroll = false;
         result.orientation = Orientation.auto;
         result.bottomTabFontFamily = new StyleParams.Font();
+        result.titleBarTitleFont = new StyleParams.Font();
+        result.titleBarSubtitleFontFamily = new StyleParams.Font();
         result.titleBarHeight = -1;
         return result;
     }