A quick fix for fallback wallpaper connection NPE.

This is a quick fix for system service crash in WallpaperManagerService
due to I didn't consider what to do if SystemUI crash or bind
ImageWallpaper failed.

Bug: 121181553
Test: Mojave boot up.
Change-Id: Ie47d9a8fdcc4140a218c5c37f566e3c2a53fb691