Calling `_cat_file` from a `ReferenceFileSystem` always returns `None`

[mpiannucci]

Reproduction:

async def test_reproduce_reference_failure():
    refs = {
        "version": 1,
        "refs": {
            "velocity/0": [
                "s3://noaa-nwm-retro-v2.0-pds/full_physics/2017/201704010000.CHRTOUT_DOMAIN1.comp",
                20942062,
                1044992,
            ],
            "velocity/1": [
                "s3://noaa-nwm-retro-v2.0-pds/full_physics/2017/201704010000.CHRTOUT_DOMAIN1.comp",
                21987054,
                992063,
            ],
            "velocity/2": [
                "s3://noaa-nwm-retro-v2.0-pds/full_physics/2017/201704010000.CHRTOUT_DOMAIN1.comp",
                42658129,
                1051879,
            ],
            "velocity/.zarray": '{"shape":[2729077],"chunks":[909693],"fill_value":-999900,"order":"C","filters":[{"id":"zlib","level":2}],"dimension_separator":".","compressor":null,"zarr_format":2,"dtype":"<i4"}',
            "velocity/.zattrs": '{"_ARRAY_DIMENSIONS":["feature_id"],"long_name":"River Velocity","units":"ms-1","coordinates":"latitude longitude","grid_mapping":"crs","missing_value":-999900,"scale_factor":0.009999999776482582,"add_offset":0.0,"valid_range":[0,50000000]}',
        },
    }
    
    fss = fsspec.filesystem(
        's3',
        anon=True,
        asynchronous=True,
    )

    fs = fsspec.filesystem(
        "reference",
        fo=refs,
        fss=fss,
        # remote_protocol="s3",
        # remote_options=dict(
        #     anon=True, default_fill_cache=False, default_cache_type="none", asynchronous=True
        # ),
        asynchronous=True,
    )

    data = await fs._cat_file("velocity/0")

# data is None