fix: use identity check for None comparison in clip_tokens tests

- Replace `== None` with `is None` in test_empty_input_text method
- Follow Python best practice for None comparisons as recommended in code review
- Address feedback from PR #1816 review comment

Co-authored-by: mrT23

tests/unittest/test_clip_tokens.py

@@ -10,7 +10,7 @@ class TestClipTokens:
     def test_empty_input_text(self):
         """Test that empty input returns empty string."""
         assert clip_tokens("", 10) == ""
-        assert clip_tokens(None, 10) == None
+        assert clip_tokens(None, 10) is None
 
     def test_text_under_token_limit(self):
         """Test that text under the token limit is returned unchanged."""