res/res_rtp_asterisk: remove debug traces generated by an empty frame

The realtime text timer pops regularly and sends text frames even if
the buffer is empty. This causes a lot of unecessary debug logging.

* Made red_write() test if we need to send a frame before calling
ast_rtp_write()

ASTERISK-28002
Reported by: Emmanuel BUU
Tested by: Emmanuel BUU

Change-Id: Icf81310c3b8080b615a42060afc02ab41f9523dd

res/res_rtp_asterisk.c

@@ -7382,7 +7382,9 @@ static int red_write(const void *data)
 	struct ast_rtp *rtp = ast_rtp_instance_get_data(instance);
 
 	ao2_lock(instance);
-	ast_rtp_write(instance, &rtp->red->t140);
+	if (rtp->red->t140.datalen > 0) {
+		ast_rtp_write(instance, &rtp->red->t140);
+	}
 	ao2_unlock(instance);
 
 	return 1;