Only show text encoding note in Differential if a change has hunks

Summary: Fixes T5503. We incorrectly render an encoding note for empty files. Only render an encoding note for text changes with at least one hunk.

Test Plan:
  - Viewed empty file, no note.
  - Viewed nonempty file with altered encoding, saw note.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5503

Differential Revision: https://secure.phabricator.com/D9780