Put default callerid into contact when the one specified is either NULL or has a zero string length. (issue #7590 reported by key2)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@38234 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -4847,7 +4847,7 @@ static void initreqprep(struct sip_request *req, struct sip_pvt *p, int sipmetho
 		l = CALLERID_UNKNOWN;
 		n = l;
 	}
-	if (!l)
+	if (ast_strlen_zero(l))
 		l = default_callerid;
 	if (ast_strlen_zero(n))
 		n = l;