Fix TypeError when faithfulness context chunk text is None

[hadbiaghiles]

Summary

FaithfulnessChecker.check() concatenates context with chunk.get("text", ""). When a chunk has the text key set to None, .get() returns None (the default only applies to missing keys), and " ".join(...) raises:

TypeError: sequence item 0: expected str instance, NoneType found

Fix

Treat text: None like a missing/empty value by joining chunk.get("text") or "". That keeps missing keys, empty strings, and None all join-safe without changing scoring for real text.

Testing

• Removed the xfail on test_none_context_chunk_text so the original crash case is expected to pass.
• Extended that test with a mixed [{"text": None}, {"text": "Knows Python well"}] chunk list to confirm None alongside real text is also join-safe.

Fixes #60