Don't pass null to manager_event()

(closes issue #17087)
Reported by: bklang
Patches:
      app-fax-null-sprintf1.diff uploaded by mnicholson (license 96)
Tested by: bklang



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@269083 65c4cc65-6c06-0410-ace0-fbb531ad65f3