Fix truthiness check on the "port" field in the IRC service hook

Empty strings are truthy and may be passed as a payload in production and would
be converted to 0, resulting in a non-working default port. The tests passed
when nil was used in the test environment.
Merge pull request #755 from gwis/hotfix/fix-irc-default-port

Fix truthiness check on the "port" field in the IRC service hook