Refactor: simplify _construct_result in emr createdefaultroles, avoid shadowing builtin list

## Summary

\`awscli/customizations/emr/createdefaultroles.py\` \`_construct_result\` (lines ~208-224) builds its result via a helper, \`_construct_role_and_role_policy_structure\`, that takes a mutable list parameter literally named \`list\` (shadowing the builtin) and mutates it in place across three repetitive calls just to conditionally append a dict.

## Proposed change

Replace the mutating helper with a list comprehension/filter over the three \`(response, policy)\` pairs:
\`\`\`python
return [
{'Role': response['Role'], 'RolePolicy': policy}
for response, policy in responses_and_policies
if response is not None and response['Role'] is not None
]
\`\`\`
This removes the builtin-shadowing parameter name and the mutating helper entirely. Pure refactor, no behavior change.

Read awscli/customizations/emr/createdefaultroles.py around _construct_result (lines ~208-224), then inspect _construct_role_and_role_policy_structure and the three response/policy calls. The work is done when the mutating helper and builtin-shadowing parameter are removed, the result is built from the three pairs, and behavior remains unchanged.

Written by the indexing model from the issue text.