Do not attempt to return file size if file does not exist, refs T79

Summary:
There is something deeper broken here... for_api should not attempt to return
the same information for a deleted media as for an active one.

Test Plan: ./bin/behat --config application/tests/behat.yml application/tests/features/api.media.feature --format progress

Reviewers: rjmackay

Reviewed By: rjmackay

Maniphest Tasks: T79

Differential Revision: https://phabricator.ushahidi.com/D52