Cellular: Fix UDPSOCKET_SENDTO_INVALID test for BG96

Author: Mirela Chirica

features/cellular/framework/targets/QUECTEL/BG96/QUECTEL_BG96_CellularStack.cpp

@@ -254,6 +254,11 @@ nsapi_size_or_error_t QUECTEL_BG96_CellularStack::socket_sendto_impl(CellularSoc
     if (size > BG96_MAX_SEND_SIZE) {
         return NSAPI_ERROR_PARAMETER;
     }
+
+    if (!size && socket->proto == NSAPI_UDP) {
+        return NSAPI_ERROR_UNSUPPORTED;
+    }
+
     int sent_len = 0;
     int sent_len_before = 0;
     int sent_len_after = 0;