res_pjsip: Update artificial auth whenever default_realm changes.

There was code attempting to update the artificial authentication object
whenever the default_realm changed.  However, once the artificial
authentication object was created it would never get updated.  The
artificial authentication object would require a system restart for a
change to the default_realm to take effect.

ASTERISK-26799

Change-Id: Id59036e9529c2d3ed728af2ed904dc36e7094802