Improve chat column behavior for newline in empty text box

Summary: Ref T7538. I got this half correct but not fully correct: when you press enter in an empty text box, do nothing (instead of: sending an empty message, or writing a literal newline).

Test Plan: Hit enter in empty chat column box.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7538

Differential Revision: https://secure.phabricator.com/D12089