Don't allow null entry to be merged

src/gui/symbology/qgscategorizedsymbolrendererwidget.cpp

@@ -1155,12 +1155,27 @@ void QgsCategorizedSymbolRendererWidget::dataDefinedSizeLegend()
 
 void QgsCategorizedSymbolRendererWidget::mergeClicked()
 {
-  QList<int> categoryIndexes = selectedCategories();
+  const QgsCategoryList &categories = mRenderer->categories();
+
+  QList<int> selectedCategoryIndexes = selectedCategories();
+  QList< int > categoryIndexes;
+
+  // filter out "" entry
+  for ( int i : selectedCategoryIndexes )
+  {
+    QVariant v = categories.at( i ).value();
+
+    if ( !v.isValid() || v == "" )
+    {
+      continue;
+    }
+
+    categoryIndexes.append( i );
+  }
+
   if ( categoryIndexes.count() < 2 )
     return;
 
-  const QgsCategoryList &categories = mRenderer->categories();
-
   QStringList labels;
   QVariantList values;
   values.reserve( categoryIndexes.count() );

tests/src/gui/testqgscategorizedrendererwidget.cpp

@@ -196,6 +196,8 @@ void TestQgsCategorizedRendererWidget::merge()
   // merging categories which are already lists
   widget->viewCategories->selectionModel()->select( widget->viewCategories->model()->index( 0, 0 ), QItemSelectionModel::ClearAndSelect | QItemSelectionModel::Rows );
   widget->viewCategories->selectionModel()->select( widget->viewCategories->model()->index( 1, 0 ), QItemSelectionModel::Select | QItemSelectionModel::Rows );
+  //"" entry should be ignored
+  widget->viewCategories->selectionModel()->select( widget->viewCategories->model()->index( 2, 0 ), QItemSelectionModel::Select | QItemSelectionModel::Rows );
   widget->mergeClicked();
 
   QCOMPARE( static_cast< QgsCategorizedSymbolRenderer * >( widget->renderer() )->categories().count(), 2 );