[Data] Fix UnboundLocalError when read_parquet with columns and no partitioning

[bveeramani]

Why are these changes needed?

If you call read_parquet with partitioning=None and non-empty columns, then Ray Data raises a error because of a missing branch. This PR fixes that issue.

Traceback (most recent call last):
  File "/root/lab42_vr/test.py", line 6, in <module>
    ds = ray.data.read_parquet(input_s3_path, partitioning=None, columns=columns)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.venv/lib/python3.12/site-packages/ray/data/read_api.py", line 950, in read_parquet
    datasource = ParquetDatasource(
                 ^^^^^^^^^^^^^^^^^^
  File "/root/.venv/lib/python3.12/site-packages/ray/data/_internal/datasource/parquet_datasource.py", line 262, in __init__
    data_columns, partition_columns = _infer_data_and_partition_columns(
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.venv/lib/python3.12/site-packages/ray/data/_internal/datasource/parquet_datasource.py", line 817, in _infer_data_and_partition_columns
    return data_columns, partition_columns
                         ^^^^^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'partition_columns' where it is not associated with a value

Related issue number

Fixes #55279

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[gemini-code-assist]

Code Review

This pull request addresses an UnboundLocalError that occurs when ray.data.read_parquet is called with partitioning=None and a non-empty columns list. The fix correctly initializes partition_columns in this scenario, preventing the error. A corresponding test case has been added to verify the fix.

My review includes one suggestion to make the new test more robust by adding an assertion to verify the content of the dataset, in addition to ensuring that no exception is raised.