Fix(chat): resolve empty-string TIN bypass masking with is not None guard

[Jean-Regis-M]

Summary

Fixes #408

Sensitive field masking skipped "" and 0 values due to Python
truthiness evaluation. Replaces falsy check with explicit is not None.

Problem

if key in result and result[key]: treats "" as False, allowing
an empty-string TIN to bypass the masking pipeline and be returned raw.
Affects tin, bank_account_number, bank_routing_number in both
VendorChatAssistant and CoPilotAssistant.

Root Cause

Truthiness guard result[key] evaluates falsy values ("", 0) as
False, skipping the masking block entirely.

Solution

Changed guard to result[key] is not None in both _call_get_vendor_details
implementations. Two-line diff, no other code touched.

Impact

• No breaking changes
• Minimal diff (2 lines across 2 locations)
• Deterministic: all non-None sensitive fields now always masked
• Zero regression risk on non-empty values

Testing

pytest tests/unit/agents/test_chat_assistant.py::TestQAFindings::test_chat_qa_002_empty_string_tin_not_masked -v
pytest tests/unit/agents/test_chat_assistant.py::TestQAFindings::test_chat_qa_003_integer_zero_tin_not_masked -v
pytest tests/unit/agents/test_chat_assistant.py -v