Avoid reparsing non-existent polls

The current poll_title reparser uses poll_max_options > 0 to determine whether a topic has a poll. However, this value does not reliably determine whether a poll exists as new topics have poll_max_options = 1 and only topics whose poll has been deleted have poll_max_options = 0.

It should use poll_start > 0 instead.

The issue is marked as trivial as it makes no functional difference whether empty poll titles are reparsed or not.