Correctly return null values from QgsColorWidgetWrapper

(cherry-picked from 94d88e6)

Author: nyalldawson

src/gui/editorwidgets/qgscolorwidgetwrapper.cpp

@@ -24,12 +24,11 @@ QgsColorWidgetWrapper::QgsColorWidgetWrapper( QgsVectorLayer* vl, int fieldIdx,
 
 QVariant QgsColorWidgetWrapper::value() const
 {
-  QVariant v;
-
+  QColor c;
   if ( mColorButton )
-    v = mColorButton->color();
+    c = mColorButton->color();
 
-  return v;
+  return c.isValid() ? QVariant( c ) : QVariant( QVariant::Color );
 }
 
 QWidget* QgsColorWidgetWrapper::createWidget( QWidget* parent )
@@ -54,5 +53,5 @@ bool QgsColorWidgetWrapper::valid() const
 void QgsColorWidgetWrapper::setValue( const QVariant& value )
 {
   if ( mColorButton )
-    mColorButton->setColor( QColor( value.toString() ) );
+    mColorButton->setColor( !value.isNull() ? QColor( value.toString() ) : QColor() );
 }