fix a bug when load empty AC comment data

comment data为空时返回new Damaku(),不返回null.
java.lang.NullPointerException
        at master.flame.danmaku.controller.CacheManagingDrawTask$CacheManager$CacheHandler.prepareCaches(Unknown Source)
        at master.flame.danmaku.controller.CacheManagingDrawTask$CacheManager$CacheHandler.handleMessage(Unknown Source)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:154)
        at android.os.HandlerThread.run(HandlerThread.java:65)