Fix a crash when uploading a ZIP to Phragment

Summary:
This crash occurs when:

  * The ZIP contains a directory entry to abc/
  * The current Phragment contains an entry to abc/

It will attempt to call createFromFile (with a null file to indicate it's a directory), but the fragment actually already exists in the database, so this will crash.

Test Plan: TBD

Reviewers: #blessed_reviewers

Subscribers: epriestley, hach-que

Differential Revision: https://secure.phabricator.com/D9001