make sure that the empty mat is copied to UMat properly - i.e. UMat becomes empty. Before the patch such copy operation crashed

modules/videoio/src/cap_avfoundation_mac.mm

@@ -118,7 +118,6 @@ private:
     int height;
     int settingWidth;
     int settingHeight;
-    OSType mInputPixelFormat;
 
     int started;
 };
@@ -163,7 +162,6 @@ private:
 
     CMTime mFrameTimestamp;
     size_t mFrameNum;
-    OSType mInputPixelFormat;
 
     int started;
 };