Allow passing empty labels in the spark kubernetes driver config

[logarithm]

"labels" section of the k8s yaml configuration is optional for both kubernetes client and spark-operator. However, if labels are not passed as part of the configuration then the job fails with the following exception:

[2025-01-21, 18:10:50 UTC] {custom_object_launcher.py:312} ERROR - Exception when attempting to create spark job
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/providers/cncf/kubernetes/operators/custom_object_launcher.py", line 294, in start_spark_job
    labels=self.spark_obj_spec["spec"]["driver"]["labels"],
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: 'labels'
[2025-01-21, 18:10:50 UTC] {taskinstance.py:3311} ERROR - Task failed with exception
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/models/taskinstance.py", line 767, in _execute_task
    result = _execute_callable(context=context, **execute_callable_kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/models/taskinstance.py", line 733, in _execute_callable
    return ExecutionCallableRunner(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/utils/operator_helpers.py", line 252, in run
    return self.func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/models/baseoperator.py", line 422, in wrapper
    return func(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/providers/cncf/kubernetes/operators/spark_kubernetes.py", line 302, in execute
    self.pod = self.get_or_create_spark_crd(self.launcher, context)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/providers/cncf/kubernetes/operators/spark_kubernetes.py", line 258, in get_or_create_spark_crd
    driver_pod, spark_obj_spec = launcher.start_spark_job(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/tenacity/__init__.py", line 336, in wrapped_f
    return copy(f, *args, **kw)
           ^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/tenacity/__init__.py", line 475, in __call__
    do = self.iter(retry_state=retry_state)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/tenacity/__init__.py", line 376, in iter
    result = action(retry_state)
             ^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/tenacity/__init__.py", line 398, in <lambda>
    self._add_action_func(lambda rs: rs.outcome.result())
                                     ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/home/airflow/.local/lib/python3.12/site-packages/tenacity/__init__.py", line 478, in __call__
    result = fn(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/providers/cncf/kubernetes/operators/custom_object_launcher.py", line 313, in start_spark_job
    raise e
  File "/home/airflow/.local/lib/python3.12/site-packages/airflow/providers/cncf/kubernetes/operators/custom_object_launcher.py", line 294, in start_spark_job
    labels=self.spark_obj_spec["spec"]["driver"]["labels"],
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: 'labels'

Proposed fix is just to use get method, which instead of KeyError would return None, which should be perfectly fine for the V1ObjectMeta object.

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[potiuk]

Can you please add a unit test showing the issue and preventing regression in the future? Thanks in advance!

[logarithm]

@potiuk - sure, I'll work on it in the next few days and update the PR. Thank you for taking a look!

[logarithm]

@potiuk - I looked at the code and how it can be unit-tested, and this is more challenging than I thought at first.

Inside the start_spark_job it calls create_namespaced_custom_object from the custom_obj_api, which makes a call to kubernetes API and returns the response object (simple dictionary).

The best option to implement the unit test was to mock the response of the custom_object_api.create_namespaced_custom_object.

@potiuk - please let me know if this is what would be a good enough test or if you have any other options in mind. Thank you for your review in advance!