R1:fb5dec4c036b

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…