res_pjsip: Fail global load if debug or default_from_user are empty

If debug was specified in the global configuration but left blank,
the logger would treat it as a wildcard and log all hosts.  If
default_from_user was empty, a crash would result.

The global apply handler now checks for empty strings.

ASTERISK-26239 #close
ASTERISK-26238 #close

Change-Id: Ie75727f5cd5808845d92cc81f5713842fb203336