Since the change of the collation functions from strxfrm to Qt's collate sorting on macOS does no longer sort empty values as expected.

See forum discussion https://community.metabrainz.org/t/picard-ignores-blank-tag-fields-instead-of-considering-them-as-null-field-when-sorting-out/734416

This only affects macOS. On Linux it works as expected, and on Windows we are using strxfrm still which also works correctly for this case.