net/udp: Support zero-length UDP datagrams

According to RFC768 page2 and referring to the Linux implementation, the message with udp length of 0 is supported.

Signed-off-by: gaohedong <gaohedong@xiaomi.com>

net/udp/udp_devpoll.c

@@ -103,7 +103,7 @@ void udp_poll(FAR struct net_driver_s *dev, FAR struct udp_conn_s *conn)
 
       /* If the application has data to send, setup the UDP/IP header */
 
-      if (dev->d_sndlen > 0)
+      if (dev->d_len > 0)
         {
           udp_send(dev, conn);
           return;