Send_Message: Fix handling of "empty" targets

Send_Message: Fix handling of "empty" targets

Clients can specify multiple targets for the "PRIVMSG", "NOTICE", and
"SQUERY" commands, separated by commas (e. g. "PRIVMSG a,#b,c :text").

Since commit 49ab79d0 ("Limit the number of message targes, and suppress
duplicates"), ngIRCd crashed when the client sent the separator character
only as target(s), e. g. "," or ",,,," etc.!

This patch fixes the bug and adds a test case for this issue.

Thanks to Florian Westphal <fw@strlen.de> for spotting the issue!