fix: return empty data instead of None when OpenLineage on_start method is missing

[kacpermuda]

When DefaultExtractor is used, so the operator has OpenLineage methods implemented (at least one of: on_start, on_complete) we are logging a WARNING each time there is no on_start method. This happens because we are validating OL metadata returned by extractor, here. This validation makes sense and we should keep it, but imho the default extractor should return empty data when there is no on_start method defined, because it means that the on_complete method will be called.

There is no need to return None ourselves and then log a warning because we found a None 😄

Some logs of current behaviour:

[2024-08-05, 15:04:30 UTC] {listener.py:383} DEBUG - Executing OpenLineage process - on_running - pid 376
[2024-08-05, 15:04:30 UTC] {manager.py:149} DEBUG - extractor for CustomOperator is <class 'airflow.providers.openlineage.extractors.base.DefaultExtractor'>
[2024-08-05, 15:04:30 UTC] {manager.py:98} DEBUG - Using extractor DefaultExtractor task_type=CustomOperator airflow_dag_id=dag task_id=task airflow_run_id=manual__2024-08-05T15:04:29.761674+00:00 
[2024-08-05, 15:04:30 UTC] {base.py:101} DEBUG - Trying to execute `get_openlineage_facets_on_start` for CustomOperator.
[2024-08-05, 15:04:30 UTC] {base.py:112} DEBUG - Operator CustomOperator does not have the get_openlineage_facets_on_start method.
[2024-08-05, 15:04:30 UTC] {manager.py:104} DEBUG - Found task metadata for operation task: None
[2024-08-05, 15:04:30 UTC] {manager.py:267} WARNING - Extractor returns non-valid metadata: None

After the change, we would get rid of this warning. We would still get the information that there is no on_start method and metadata found would be OperatorLineage()

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