[3.13] gh-124486: Fix test_whichdb_ndbm in test_dbm on NetBSD (GH-136335) (GH-136379)

On NetBSD, ndbm.open() does not fail for empty file.
(cherry picked from commit b7aa2a4b4df697db6ea45a555eeb3fefa5ca5bd4)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>