Add test for chunker.chunk(None) to cover guard clause

tests/test_chunker.py

@@ -55,6 +55,13 @@ async def test_local_chunker(qa_corpus: list[dict[str, str]]):
     assert abs(total_tokens - original_tokens) <= original_tokens * 0.1
 
 
+@pytest.mark.asyncio
+async def test_local_chunker_none_document():
+    """Test DoclingLocalChunker returns empty list for None document."""
+    chunker = DoclingLocalChunker()
+    assert await chunker.chunk(None) == []
+
+
 @pytest.mark.asyncio
 async def test_local_chunker_custom_config():
     """Test DoclingLocalChunker with custom configuration."""