chan_pjsip: Fix an assertion for channels that lack formats on creation

ASTERISK-24222 #close
Reported by: Mark Michelson
Review: https://reviewboard.asterisk.org/r/4017/



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@424333 f38db490-d61c-443f-a65b-d21fe96a405b