Contact import (new) deletes contact fields

Overview

It seems like the "new importer" that has been implemented with 5.51 has slightly changed its behaviour: if you import/update contacts with empty fields these fields will be deleted, where they used to be ignored (pre 5.51).

Reproduction steps

1. Import test contact using the "Import Contacts" menu item with this file and check whether the contact was created.
2. Update the contact by using the import again with this file, setting the "For Duplicate Contacts" setting to "Update"
3. Observe, that the birthday field in the new contact was deleted.

Current behaviour

Currently, the contact's birthday field is deleted.

Expected behaviour

Contact's birthday field should be left untouched, as was the behaviour pre 5.51.0 (tested with 5.40.2).

Environment information

Reproduced on dmaster.

---

This might be an intentional change in behaviour, but I haven't found any documentation on this.