If transformer set on property, allow transformer to handle null values

src/core/qgsproperty.cpp

@@ -464,11 +464,13 @@ QVariant QgsProperty::value( const QgsExpressionContext& context, const QVariant
 
   bool valOk = false;
   QVariant val = propertyValue( context, defaultValue, &valOk );
-  if ( !valOk )
+  if ( !d->transformer && !valOk ) // if transformer present, let it handle null values
     return defaultValue;
 
   if ( d->transformer )
   {
+    if ( !valOk )
+      val = QVariant();
     val = d->transformer->transform( context, val );
   }
 

tests/src/core/testqgsproperty.cpp

@@ -807,6 +807,26 @@ void TestQgsProperty::sizeScaleTransformer()
   t2.setType( QgsSizeScaleTransformer::Exponential );
   t2.setExponent( 1.6 );
   QCOMPARE( t2.toExpression( "5+6" ), QStringLiteral( "coalesce(scale_exp(5+6, 15, 25, 150, 250, 1.6), -10)" ) );
+
+  // test size scale transformer inside property
+  QgsProperty p;
+  p.setTransformer( new  QgsSizeScaleTransformer( QgsSizeScaleTransformer::Exponential,
+                    15,
+                    25,
+                    150,
+                    250,
+                    -10,
+                    99 ) );
+  p.setStaticValue( QVariant() );
+  bool ok = false;
+  QCOMPARE( p.valueAsDouble( context, 100, &ok ), -10.0 );
+  QVERIFY( ok );
+  p.setExpressionString( QStringLiteral( "NULL" ) );
+  QCOMPARE( p.valueAsDouble( context, 100, &ok ), -10.0 );
+  QVERIFY( ok );
+  p.setExpressionString( QStringLiteral( "no field" ) );
+  QCOMPARE( p.valueAsDouble( context, 100, &ok ), -10.0 );
+  QVERIFY( ok );
 }
 
 void TestQgsProperty::sizeScaleTransformerFromExpression()