No reason to expected ufunc.types_ to be None.

Example where that assumption is wrong is the following test invocation:

```
python -m pytest \
   dpctl/tests/test_tensor_*.py \
   dpctl/tests/test_usm_ndarray_*.py \
   dpctl/tests/elementwise/test_a*.py
```

Author: oleksandr-pavlyk

dpctl/tests/elementwise/test_abs.py

@@ -73,7 +73,7 @@ def test_abs_usm_type(usm_type):
 def test_abs_types_prop():
     get_queue_or_skip()
     types = dpt.abs.types_
-    assert types is None
+    assert types is None or isinstance(types, list)
     types = dpt.abs.types
     assert isinstance(types, list)
     assert len(types) > 0

dpctl/tests/elementwise/test_add.py

@@ -261,7 +261,7 @@ def __sycl_usm_array_interface__(self):
 def test_add_types_property():
     get_queue_or_skip()
     types = dpt.add.types_
-    assert types is None
+    assert types is None or isinstance(types, list)
     types = dpt.add.types
     assert isinstance(types, list)
     assert len(types) > 0