no-infer fixup

pandas/tests/series/indexing/test_setitem.py

@@ -599,7 +599,7 @@ def test_setitem_enlargement_object_none(self, nulls_fixture, using_infer_string
 
  expected = Series(["a", "b", nulls_fixture], index=[0, 1, 3], dtype=dtype)
  tm.assert_series_equal(ser, expected)
- if using_infer_string:
+ if dtype == "str":
  ser[3] is np.nan
  else:
  assert ser[3] is nulls_fixture