Make check for non-existant channel more correct

Currently, this doesn't change anything; all channels start with '#'.
However, when pull #107 (Improve irc-msg support for multiple
connections) lands we want to accept, e.g., '0' as a valid user name.
So we should actually be checking if the value is defined, rather than
if it's not falsy.  I've also excluded empty strings as valid user
names/channels, which I'm not sure is the right decision.