Fix TypeError in s3 error handler when Error.Message is None

[omghante]

Description

Fix TypeError: argument of type 'NoneType' is not iterable in awscli/customizations/s3errormsg.py when S3-compatible endpoints return error responses with empty <Message></Message> XML elements.

Fixes #10161

Root Cause

S3-compatible endpoints (Ceph RadosGW, Hetzner, MinIO) may return valid XML error responses with empty <Message> elements. Python's ElementTree parses empty elements as None (i.e., .text = None), and botocore's error parsing path in _replace_nodes() does not normalize None to "". This causes the parsed response dict to contain {'Error': {'Message': None}}.

The _is_sigv4_error_message() and _is_kms_sigv4_error_message() functions then perform 'substring' in None, which raises:

TypeError: argument of type 'NoneType' is not iterable

Key Python gotcha: .get('Message', '') returns the default '' only when the key is absent. When the key exists with value None, .get() returns None.

Changes

• Use or '' coalescing in _is_sigv4_error_message(), _is_permanent_redirect_message(), and _is_kms_sigv4_error_message() to handle None values safely
• Guard enhance_error_msg() body against None Message and missing Endpoint in the PermanentRedirect code path
• Add 6 unit tests covering None Message, None Code, empty string Message, and PermanentRedirect with None Message

Testing

• All 5 existing tests pass without modification zero regressions
• 6 new tests validate the fix across all vulnerable code paths
• pytest tests/unit/customizations/test_s3errormsg.py -v 11 passed
• No behavioral change for AWS S3 (AWS always returns non-empty Message content)

Credit

Root cause analysis by @sfwester in #10161.