PJSIP: Backport r405270 - Unhold on reinvite without SDP

Adds behavior to unhold on a reinvite without an SDP section
Review: https://reviewboard.asterisk.org/r/3106/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@405350 f38db490-d61c-443f-a65b-d21fe96a405b