mod_muc_rtbl: Optimize case with zero hashes On the assumption that during quiet times between torrents of spam, the hash set would be empty. There would be no point in doing the operations and hashes to check for a match in that case.

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.