fix: treat a .pyiceberg.yaml without a mapping as no config (#3915)

`_load_yaml` passed `strictyaml.load(...).data` straight to
`_lowercase_dictionary_keys`. For an empty or comment-only document that
value is a `str`, not a mapping, so the call raised
`AttributeError: 'str' object has no attribute 'items'`.

`Config()` runs at import time, so commenting out the file made
`import pyiceberg.catalog` fail with an error naming neither YAML nor the
file. Return `None` instead, which the annotated return type already allows
and which the caller already handles as "keep looking".

Co-authored-by: Claude Code <noreply@anthropic.com>