obs-ffmpeg: Fix frame remain after changing file

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After changing the media source from video file to audio file,
the last video's frame (preloaded) maybe still rendered.

This commit ensures that when media source changed to
another one without video, clean the source's video output,
by simply add judgement to check whether the media has video.

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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f