Fix warning in tests

[romanc]

Description

Fixes

tests/test_halo_update.py::test_halo_updater_stability[interface_3d-numpy-layout0-2-more-0]
  [...]/NDSL/.venv/lib/python3.11/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <function HaloUpdater.__del__ at 0x7f74b5b91260>
  
  Traceback (most recent call last):
    File "[...]/NDSL/ndsl/halo/updater.py", line 88, in __del__
      transformer.finalize()
    File "[...]/NDSL/ndsl/halo/data_transformer.py", line 228, in finalize
      Buffer.push_to_cache(self._pack_buffer)
    File "[...]/NDSL/ndsl/buffer.py", line 70, in push_to_cache
      BUFFER_CACHE[buffer._key].append(buffer)
                   ^^^^^^^^^^^
  AttributeError: 'NoneType' object has no attribute '_key'
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

as seen e.g. in https://github.com/NOAA-GFDL/NDSL/actions/runs/15278670407/job/42971686345?pr=150#step:6:773

A bit more context on when/why this happens: The key error comes from test_halo_updater_stability() manually calling transformer.finalize() before finalize() is called again from the HaloUpdater's __del__ function. In "normal" execution mode, this shouldn't be a problem because developers aren't supposed to manually call finalize() on the transformers.

An alternative way to avoid the error could be to add a finalized: bool state to HaloDataTransformer and exit early in finalize() in case the transformer has already been finalized.

How Has This Been Tested?

Running tests locally

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[FlorianDeconinck]

That tracks, the del routine still catches missing wait