mod_admin_blocklist: Fix traceback due to misplaced parenthesis
get_jids_with_role() throws an exception when 'host' is nil since
Prosody trunk rev 7ca5645f46cd
This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).