Now MapStyle.CLEAR_BACKGROUND_IMAGE is a String and not null. It can not work well because of many reasons: 

1. use of comparison operator in line if (file == MapStyle.CLEAR_BACKGROUND_IMAGE) in method MapView.loadBackgroundImage can not return true if the String comes from the properties because it compares instances not values. 

2. If background image is not set the property MapStyle.RESOURCES_BACKGROUND_IMAGE should better not be set to avoid confusions.