fix addpart tool warning

The isGeometryEmpty variable was true when geometry was present and viceversa.
Thus the warning for multiple parts for single geometry was giving false positives and
negatives.

src/app/qgsmaptooladdpart.cpp

@@ -71,7 +71,7 @@ void QgsMapToolAddPart::cadCanvasReleaseEvent( QgsMapMouseEvent *e )
     QgsFeatureIterator selectedFeatures = vlayer->getSelectedFeatures();
     QgsFeature firstSelectedFeature;
     if ( selectedFeatures.nextFeature( firstSelectedFeature ) )
-      if ( !firstSelectedFeature.geometry().isNull() )
+      if ( firstSelectedFeature.geometry().isNull() )
         isGeometryEmpty = true;
   }