Sync test fails when server doesn't have signatures/marks expected by client

[rochacbruno]

Problrm introduced by #1326

stage sync tests fail when syncing a PAH from master:

ERROR    galaxy_ng.tests.integration.utils.tasks:tasks.py:30 {'traceback': '  File "/usr/local/lib/python3.8/site-
packages/pulpcore/tasking/pulpcore_worker.py", line 453, in _perform_task\n    result = func(*args, **kwargs)\n  File 
"/usr/local/lib/python3.8/site-packages/pulp_ansible/app/tasks/collections.py", line 191, in sync\n    repo_version = 
d_version.create()\n  File "/usr/local/lib/python3.8/site-packages/pulpcore/plugin/stages/declarative_version.py", line 161, in
 create\n    loop.run_until_complete(pipeline)\n  File "/usr/lib64/python3.8/asyncio/base_events.py", line 616, in 
run_until_complete\n    return future.result()\n  File "/usr/local/lib/python3.8/site-packages/pulpcore/plugin/stages/api.py",
 line 225, in create_pipeline\n    await asyncio.gather(*futures)\n  File "/usr/local/lib/python3.8/site-
packages/pulpcore/plugin/stages/api.py", line 43, in __call__\n    await self.run()\n  File "/usr/local/lib/python3.8/site-
packages/pulp_ansible/app/tasks/collections.py", line 1023, in run\n    await asyncio.gather(*tasks)\n  File 
"/usr/local/lib/python3.8/site-packages/pulp_ansible/app/tasks/collections.py", line 817, in _fetch_collection_metadata\n   
 await self._fetch_paginated_collection_metadata(\n  File "/usr/local/lib/python3.8/site-
packages/pulp_ansible/app/tasks/collections.py", line 760, in _fetch_paginated_collection_metadata\n    await 
asyncio.gather(*tasks)\n  File "/usr/local/lib/python3.8/site-packages/pulp_ansible/app/tasks/collections.py", line 573, in 
_fetch_collection_version_metadata\n    await self._add_collection_version(api_version, collection_version_url, metadata)\n  File
 "/usr/local/lib/python3.8/site-packages/pulp_ansible/app/tasks/collections.py", line 650, in _add_collection_version\n    for 
mark_value in marks:\n', 'description': "'NoneType' object is not iterable"}

Client running pulp_ansible main expects fields signatures and marks however the server doesn't have one of the fields yet (older version).

The current problem is on marks but is is a potential problem with signatures also, so the fix might include default value for both.