Fix pasting unsaved changes as temporary scratch layers

[uclaros]

Description

Pasting unsaved changes as a temporary scratch layer could fail when source is a db.
Geopackages would have Autogenerate as an fid value, Postgres layers would have nextval(... values etc, which could not be stored in the target int field.

Instead of failing, this PR continues with a null value instead.

Fixes #38913

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 1a0d81d)

[nyalldawson]

Does this work if you modify QgsField::convertCompatible to add something like this just after the initial null check?

  if ( QgsVariantUtils::isNull( v ) )
  {
    v.convert( d->type );
    return true;
  }

  if ( v.userType() == QMetaType::type( "QgsUnsetAttributeValue" ) )
  {
    return true;
  }

[uclaros]

Autogenerate and nextval(...) are of userType() == QMetaType::Type::QString, so that will not work.

[nyalldawson]

@uclaros

Autogenerate and nextval(...) are of userType() == QMetaType::Type::QString,

They shouldn't be! Are you checking the field type or value type?

[uclaros]

Are you checking the field type or value type?

Value type is string, field type should be int.

I can see QgsUnsetAttributeValue is only used when splitting and duplicating features. It's not clear to me how this class should be used.