[rbp/omxplayer] Reset display region when video stream properties change

Currently when video stream properties change, e.g. when user switches to next or previous Live TV channel, and has the same frame width and height, the COMXVideo::SetVideoRect() method is not called, causing the video to play back with default source and destination rectangles, not respecting the user-defined zoom settings.

This commit fixes the issue.
FIX: [AE] if null sink is requested, return null sink