fix: getSnapshot method to return null instead of an empty string when textureView is null

Author: romaingyh

flutter_vlc_player/android/src/main/java/software/solid/fluttervlcplayer/FlutterVlcPlayer.java

@@ -637,7 +637,7 @@ void castToRenderer(String rendererDevice) {
 
     @Nullable
     String getSnapshot() {
-        if (textureView == null) return "";
+        if (textureView == null) return null;
 
         Bitmap bitmap = textureView.getBitmap();
         if (bitmap == null) return null;