Handle the case when the video contains frames with no face

[jhj0517]

Related issues / PRs.

• 'NoneType' object has no attribute 'savefig' #9

Summarize Changes

1. Handle the exception case that there might be frames with no face. Even if there are no faces at all in the video, it still needs to go through the pipeline because pipeline resize the frames of the video.
   If the video contains both frames with faces and no faces, one side has to be resized to other side, because the end result should be a video where all frames are the same size.
   So just pass to the pipeline to resize the frames even if there's no face at all.