[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 b7aa2a4)

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

Author: miss-islington

Lib/test/test_dbm.py

@@ -213,7 +213,8 @@ def test_whichdb(self):
     @unittest.skipUnless(ndbm, reason='Test requires ndbm')
     def test_whichdb_ndbm(self):
         # Issue 17198: check that ndbm which is referenced in whichdb is defined
-        with open(_fname + '.db', 'wb'): pass
+        with open(_fname + '.db', 'wb') as f:
+            f.write(b'spam')
         _bytes_fname = os.fsencode(_fname)
         fnames = [_fname, os_helper.FakePath(_fname),
                   _bytes_fname, os_helper.FakePath(_bytes_fname)]