This update returns None instead of empty FrameContainer in the preprocessor Wrapper class for an extreme case (for example, wrapped preprocessor returned None for all frames in video for some reasons).

Works well with --allow-missing-files option, because preprocessed file will not be written in this case.

Currently, empty FrameContainer is written to file, which might cause an error in the Extractor, if not handled. Also, added the test for this case.

@mguenther or @amohammadi, please review and merge if OK with you. Thank you!

@ageorge, can be useful for you as-well.