[net][at] Add AT serial device receive data length judgment.

components/net/at/src/at_client.c

@@ -577,7 +577,11 @@ static void client_parser(at_client_t client)
 
 static rt_err_t at_client_rx_ind(rt_device_t dev, rt_size_t size)
 {
-    rt_sem_release(at_client_local->rx_notice);
+    if (size > 0)
+    {
+        rt_sem_release(at_client_local->rx_notice);
+    }
+
     return RT_EOK;
 }
 

components/net/at/src/at_server.c

@@ -433,7 +433,10 @@ __retry:
 
 static rt_err_t at_rx_ind(rt_device_t dev, rt_size_t size)
 {
-    rt_sem_release(at_server_local->rx_notice);
+    if (size > 0)
+    {
+        rt_sem_release(at_server_local->rx_notice);
+    }
 
     return RT_EOK;
 }