qdrant: Filters don't function properly when only the key attribute is specified

Current Behavior

When you only specify a filter key but not a match or a range, the filter will always return an empty set. It seems like the filter tries to match against null which would explain the result for specified payloads.

Expected Behavior

I would expect empty rules to be ignored if not specified, so that the filter ambiguously returns every point that contains the key.

Steps to Reproduce

openapi/tests/openapi_integration/helpers/collection_setup.py:

                {
                    "id": 1,
                    "vector": [0.05, 0.61, 0.76, 0.74],
                    "payload": {"city": "Berlin", "population": 4500000}
                },

openapi/tests/openapi_integration/test_filter.py:

def test_match_key():
    response = request_with_validation(
        api='/collections/{collection_name}/points/search',
        method="POST",
        path_params={'collection_name': collection_name},
        body={
            "vector": [0.2, 0.1, 0.9, 0.7],
            "limit": 3,
            "filter": {
                "must": [
                    {
                        "key": "population",
                    }
                ]
            }
        }
    )
    assert response.ok

    print(response.json())

    json = response.json()
    assert len(json['result']) == 1

    ids = [x['id'] for x in json['result']]
    assert 1 in ids