Handle list_object_v2 calls with missing KeyCount
Handle list_object_v2 calls with missing KeyCount

Some s3 server implementations such as OpenStack Object Storage
("Swift") don't provide the field KeyCount in the response to list_object_v2.

This change handles this gracefully and reverts to checking other keys
("Contents" and "CommonPrefixes").

See https://github.com/dask/s3fs/issues/404 for more details.

fsspec/s3fs