chan_pjsip: don't use PJSIP_SC_NULL as it only exists pjproject 2.8+

A patch made a reference to the PJSIP_SC_NULL enumeration value, which
was added to pjproject 2.8 and above thus making it so Asterisk would
fail to compile with prior versions of pjproject.

This patch removes the reference, and instead initializes the value
to '0'.

ASTERISK-28886 #close

Change-Id: I68491c80da1a0154b2286c9458440141c98db9d7

channels/chan_pjsip.c

@@ -1993,7 +1993,7 @@ static void xfer_client_on_evsub_state(pjsip_evsub *sub, pjsip_event *event)
 		/* Check for NOTIFY complete or error. */
 		pjsip_msg *msg;
 		pjsip_msg_body *body;
-		pjsip_status_line status_line = { .code = PJSIP_SC_NULL };
+		pjsip_status_line status_line = { .code = 0 };
 		pj_bool_t is_last;
 		pj_status_t status;