MultiFileField should return an empty list by default.

Before, it would return either a list of UploadedFiles or None.
However, this makes it harder to iterate over the results, for example,
the following line (taken from the docstring) fails if no files are
submitted:

    for media_file in form.cleaned_data['field_name']:
        MyMedia.objects.create(
            name=media_file.name,
            file=media_file,
        )

Will result in a "TypeError: 'NoneType' object is not iterable".  If you
return an empty list instead, the code example won't fail if no files
are submitted.

cc @chroto