Launcher: silence 'Unknown aircraft URI scheme' warning on empty selection

------------------------------------------------------------------------
If no aircraft is selected in the launcher, the routine
AircraftItemModel::indexOfAircraftURI is called with an empty QUri,
triggering a warning in the terminal.

This commit removes such warning by ignoring QUris with empty schemes
(the routine still returns an invalid index).

src/GUI/AircraftModel.cxx

@@ -862,6 +862,8 @@ QModelIndex AircraftItemModel::indexOfAircraftURI(QUrl uri) const
                 }
             } // of linear package scan
         }
+    } else if (uri.scheme() == "") {
+        // Empty URI scheme (no selection), nothing to do
     } else {
         qWarning() << "Unknown aircraft URI scheme" << uri << uri.scheme();
     }