Add a linter rule for detecting empty files

Summary: Adds two different linter rules (one general purpose and another PHP specific) to prevent empty files from being added to a repository. For some unknown reason, people seem to like to do this.

Test Plan: Added test cases.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: avivey, cburroughs, Korvin

Differential Revision: https://secure.phabricator.com/D13881