Change match criteria existence in ast_channel_cmp_cb to use ast_strlen_zero.

(closes issue #16161)
Reported by: may213
Patches: 
      core-show-channel.patch uploaded by may213 (license 454)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@235226 65c4cc65-6c06-0410-ace0-fbb531ad65f3