Fix for ThumbnailError when input file is None

[alurin]

When first argument of get_thumbnail is None, original ThumbnailBackend backend return dummy or None.

This commit is fixed this.

[aidanlister]

Hey alurin, thanks for using the module! I can't test this at the moment, but it seems like this would return a dummy image rather than returning the full unscaled image which might not be as desirable?

[alurin]

It's only depends on sorl thumbnails settings for dummy thumbs. I think it's a good idea to return dummy image, if it's require by user settings.