fix: omit Comment key when column_comment is None in add_column

[hsusul]

Description

This PR fixes a ParamValidationError in awswrangler.catalog.add_column() when calling the function without providing a column_comment (leaving it as default column_comment=None).

Root Cause

When column_comment is None, add_column() appends {"Name": column_name, "Type": column_type, "Comment": column_comment} into table_input["StorageDescriptor"]["Columns"].
During the subsequent client_glue.update_table call, botocore validates TableInput against the AWS Glue service model. Because "Comment": None is explicitly set, botocore parameter validation fails:
botocore.exceptions.ParamValidationError: Parameter validation failed: Invalid type for parameter TableInput.StorageDescriptor.Columns[X].Comment, value: None, type: <class 'NoneType'>, valid types: <class 'str'>

Fix

Construct the column dictionary dynamically so that the "Comment" key is only included when column_comment is not None.

Regression Tests & Validation

• Added test_glue_add_column_without_comment in tests/unit/test_moto.py testing add_column both with default column_comment=None and with an explicit comment string.
• Ran AWS_DEFAULT_REGION=us-east-1 uv run pytest tests/unit/test_moto.py (47 passed).
• Ran uv run ruff check awswrangler/catalog/_add.py tests/unit/test_moto.py.
• Ran uv run mypy awswrangler/catalog/_add.py.
• Verified git diff --check.
• Live AWS integration tests were omitted as local moto unit tests provide full, deterministic verification without requiring live AWS credentials.