fix bedrock pii redaction null value handling

[ryangoldblatt-bm]

Fix the following error when applying Bedrock PII guardrails:

{"message": "Guardrail log redaction failed: 'NoneType' object is not iterable", "level": "WARNING", "timestamp": "2026-02-24T12:50:53.163260"}

Basically the current processing of the Bedrock API response does not handle null values gracefully, e.g. Bedrock API response snippet:

'sensitiveInformationPolicy': {'piiEntities': [{'action': 'NONE', 'detected': True, 'match': 'joebloggs@gmail.com', 'type': 'EMAIL'}], 'regexes': None}, 'topicPolicy': None, 'wordPolicy': None}],

[Quentin-M]

Just merge away mate, it's a working repo.