Immediately deactivate remarkup autocomplete if there's no query

Summary:
Fixes T12479. If you end a line with a character like ":" in a context which can trigger autocomplete (e.g., .:), then try to make a newline, we swallow the keystroke.

Instead, allow the keystroke through if the user hasn't typed anything else yet.

Test Plan:

• Autocompleted emoji and users normally.
• In an empty textarea, typed .:<return>, got a newline instead of a swallowed keystroke.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12479

Differential Revision: https://secure.phabricator.com/D17583