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
Introduction
============
This module checks the IP addresses attempting to register an account
against a DNSBL, blocking the attempt if there is a hit.
Configuration
=============
Option Type Default
------------------- -------- ------------
registration\_rbl string *Required*
Compatibility
=============
Prosody Trunk
[1a0b76b07b7a](https://hg.prosody.im/trunk/rev/1a0b76b07b7a) or later.