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: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.