Resolve unit test failure that occurred on Mac OSX.

On Linux (glibc), regcomp() does not return an error for an empty string.
However, the version on OSX will return an error.  The test for channel group
matching by regex now passes on the mac, as well.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@252241 65c4cc65-6c06-0410-ace0-fbb531ad65f3