MDL-49025 assign: Submissions of an empty directory should not save

MDL-49025 assign: Submissions of an empty directory should not save

Before this change if only a folder was submitted the submission_is_empty
check would return true. This meant that the draft area would be saved.
The is_empty check would then be called which would return false. This
caused the submission form to indicate to the user that the submission
had failed.

Now is_empty and submission_is_empty should both return false if only
folders are submitted.