Fix string representations for untyped parameters

[Old-Ding]

Summary

• handle the default value_type=None in parameter description string representations
• preserve existing type names when an explicit type is configured
• cover both ParameterValue and Parameter

Testing

• focused source behavior probe: both baseline classes raise AttributeError, while the fixed classes render the default type as None
• isolated class-level probes cover None, scalar int / str, and all four supported typing.List[...] value types
• ament_flake8, ament_pep257, and ament_copyright on both modified files
• python -m py_compile on both modified files
• git diff --check
• repository-native pytest is unavailable on this host because the compiled lifecycle_msgs package is not installed

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

• Queue this pull request