converted size expr evaluate to 0 on NULL field

fix #12888
use coalesce(expr,0) to have zero size if expression evaluates to NULL
otherwise the default size would be used

src/core/symbology-ng/qgsrendererv2.cpp

@@ -665,20 +665,20 @@ void QgsFeatureRendererV2::convertSymbolSizeScale( QgsSymbolV2 * symbol, QgsSymb
     QgsMarkerSymbolV2 * s = static_cast<QgsMarkerSymbolV2 *>( symbol );
     if ( QgsSymbolV2::ScaleArea == QgsSymbolV2::ScaleMethod( method ) )
     {
-      const QgsDataDefined dd( "sqrt(" + QString::number( s->size() ) + " * (" + field + "))" );
+      const QgsDataDefined dd( "coalesce(sqrt(" + QString::number( s->size() ) + " * (" + field + ")),0)" );
       s->setDataDefinedSize( dd );
     }
     else
     {
-      const QgsDataDefined dd( QString::number( s->size() ) + " * (" + field + ")" );
+      const QgsDataDefined dd( "coalesce(" + QString::number( s->size() ) + " * (" + field + "),0)" );
       s->setDataDefinedSize( dd );
     }
     s->setScaleMethod( QgsSymbolV2::ScaleDiameter );
   }
   else if ( symbol->type() == QgsSymbolV2::Line )
   {
     QgsLineSymbolV2 * s = static_cast<QgsLineSymbolV2 *>( symbol );
-    const QgsDataDefined dd( QString::number( s->width() ) + " * (" + field + ")" );
+    const QgsDataDefined dd( "coalesce(" + QString::number( s->width() ) + " * (" + field + "),0)" );
     s->setDataDefinedWidth( dd );
   }
 }