Require valid comments to contain at least one non-whitespace character

Summary:
See downstream https://phabricator.wikimedia.org/T88655. This is very marginal, but we currently allow comments consisting of only whitespace.

These are probably always mistakes, so treat them like completely empty comments.

(We intentionally do not trim leading or trailing whitespace from comments when posting them becuase leading spaces can be used to trigger codeblock formatting.)

Test Plan:

• Posted empty, nonempty, and whitespace-only comments.
• Whitespace-only comments now have the same behavior as truly empty comments (e.g., do not actually generate a transaction).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20562