res_pjsip_caller_id: Fix crash on session timers UPDATE on inbound calls.

The res_pjsip_caller_id module wrongly assumed that a
saved From header would always exist on sessions. This
is true until an inbound call is received and a session
timer causes an UPDATE to be sent. In this case there will
be no saved From header and a crash will occur. This change
makes it fall back to the From header of the outgoing request
if no saved From header is present.

ASTERISK-26307 #close

Change-Id: Iccc3bc8d243b5ede9b81abf960292930c908d4fa