apply_neighborhood: overlap should be optional

[soxofaan]

in apply_neighborhood: overlap should be optional, but if user does not specify it, it currently false with obscure

OpenEoApiError: [500] Internal: Server error: TypeError("'NoneType' object is not iterable") (ref: r-2505081220274fdeb7ee61f63c81bbd8)

from logs:

File "/opt/openeo/lib/python3.8/site-packages/openeo_driver/ProcessGraphDeserializer.py", line 1871, in apply_process
    return process_function(args=ProcessArgs(args, process_id=process_id), env=env)
  File "/opt/openeo/lib/python3.8/site-packages/openeo_driver/ProcessGraphDeserializer.py", line 935, in apply_neighborhood
    return data_cube.apply_neighborhood(process=process, size=size, overlap=overlap, env=env, context=context)
  File "/opt/openeo/lib/python3.8/site-packages/openeo_driver/dry_run.py", line 857, in apply_neighborhood
    overlap_dict = {e["dimension"]: e for e in overlap}
TypeError: 'NoneType' object is not iterable