Allow znc-only users to join partyline channels

As it stands,  users that are not connected to an external network cannot join partyline channels.   Here's how I fixed it on my own bouncer.

I openly admit that this patch is a kludge,  but the approach taken didn't require me to delve deeply into the ZNC source.   There are certainly better ways to fix this,  so I'm not particularly expecting this pull request to be accepted,  but rather I'm offering it as documentation of the issue.