Return invalid fid instead of null fid

The default comparison code for two QVariants does a really crappy job
when it comes to NULL values.
I.e. it will return true for `QVariant( 0 ) == QVariant( QVariant::Int )`
We work around this issue here by returning an invalid QVariant instead,
which - and luckily also Qt is aware of this - also is different from 0.

Fix #16745

src/gui/attributetable/qgsfeaturelistmodel.cpp

@@ -75,10 +75,6 @@ QVariant QgsFeatureListModel::data( const QModelIndex &index, int role ) const
     {
       return QSettings().value( "qgis/nullValue", "NULL" ).toString();
     }
-    else if ( role == QgsAttributeTableModel::FeatureIdRole )
-    {
-      return QVariant( QVariant::Int );
-    }
     else
     {
       return QVariant( QVariant::Invalid );