do not send empty lists

src/PlugIn.cpp

@@ -470,9 +470,11 @@ void PlugIn::OnTimer(int counter) {
         }
 
         /* send the report */
-        auto sequence = Backend::instance().update(update);
-        if (true == inserted && nullptr != sequence)
-            this->DisplayUserMessage(PLUGIN_NAME, "ERROR", "Unable to send a new aircraft report update", true, true, true, true, true);
+        if (true == inserted) {
+            auto sequence = Backend::instance().update(update);
+            if (nullptr != sequence)
+                this->DisplayUserMessage(PLUGIN_NAME, "ERROR", "Unable to send a new aircraft report update", true, true, true, true, true);
+        }
 
         airport.second.clear();
     }