Treat empty config subsection values as empty dicts

[norus]

Description

When a config file contains an empty subsection key such as s3 = with no value or indented block following it, the INI parser returns an empty string. Downstream code that calls .get('s3', {}).get(...) on the parsed config raises AttributeError because str has no .get() method, producing the cryptic message:

'str' object has no attribute 'get'

This error surfaces for any AWS CLI command, not just S3, because botocore checks the s3 config for dualstack endpoint settings during client construction.

Fix

In raw_config_parse(), when parse_subsections is enabled and a config value is an empty string, treat it as an empty dict {} rather than storing the raw empty string. This matches the logical meaning of an empty subsection block.

Reproduction

[default]
region=us-west-2
s3=
endpoint_url=http://localhost:1234/

$ aws sts get-caller-identity
'str' object has no attribute 'get'

After fix, the command proceeds normally (no config parse error for the empty s3 block).

Testing

• Added test config file aws_config_nested_empty
• Added test_nested_hierarchy_with_empty_subsection: verifies empty s3= produces {}
• Added test_nested_hierarchy_empty_subsection_allows_get: verifies downstream .get() chain works
• All 19 configloader tests pass

Fixes aws/aws-cli#9469