Don't pass zero length callerid to ooh323 stack

Don't pass zero callerid string to ooh323 stack because it can't encode this properly and
can't generate setup message.

(closes issue #17186)
Reported by: vmikhelson
Patches:
      zero_callerid_num.patch uploaded by may213 (license 454)
Tested by: may213



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@258838 65c4cc65-6c06-0410-ace0-fbb531ad65f3