[labels] Fix null data defined value was not ignored in some cases

Cherry-picked from eb6279a

Author: nyalldawson

src/app/qgslabelpropertydialog.cpp

@@ -219,7 +219,7 @@ void QgsLabelPropertyDialog::setDataDefinedValues( const QgsPalLayerSettings &la
     }
 
     QVariant result = layerSettings.dataDefinedValue( propIt.key(), mCurLabelFeat, vlayer->pendingFields() );
-    if ( !result.isValid() )
+    if ( !result.isValid() || result.isNull() )
     {
       //could not evaluate data defined value
       continue;

src/core/qgspallabeling.cpp

@@ -1205,7 +1205,7 @@ bool QgsPalLayerSettings::dataDefinedEvaluate( DataDefinedProperties p, QVariant
 
   QVariant result = dataDefinedValue( p, *mCurFeat, *mCurFields );
 
-  if ( result.isValid() ) // filter NULL values? i.e. && !result.isNull()
+  if ( result.isValid() && !result.isNull() )
   {
     //QgsDebugMsgLevel( QString( "result type:" ) + QString( result.typeName() ), 4 );
     //QgsDebugMsgLevel( QString( "result string:" ) + result.toString(), 4 );