…rking directory

Let's say if `lpath` is `file` literal, a relative path and to be downloaded to
the current working directory, `_get_file` currently fails with `FileNotFoundError`
during `os.makedirs` call.
```python
await fs._get_file('gs://bucket/file', 'file')
```

This happens because `os.path.dirname()` returns an empty string, which when passed to `os.makedirs()`,
it raises a `FileNotFoundError`.

```py
>>> os.path.dirname('file')
''
>>> os.makedirs('')
FileNotFoundError: [Errno 2] No such file or directory: ''
```

The following is a part of a traceback that I get.

```pytb
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/fsspec/asyn.py", line 118, in wrapper
    return sync(self.loop, func, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/fsspec/asyn.py", line 103, in sync
    raise return_result
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/fsspec/asyn.py", line 56, in _runner
    result[0] = await coro
                ^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/gcsfs/core.py", line 1491, in _get_file
    await self._get_file_request(u2, lpath, callback=callback, **kwargs)
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/decorator.py", line 221, in fun
    return await caller(func, *(extras + args), **kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/gcsfs/retry.py", line 123, in retry_request
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/user/dev/solutions/checkpoints-gcp/.venv/lib/python3.11/site-packages/gcsfs/core.py", line 1472, in _get_file_request
    os.makedirs(os.path.dirname(lpath), exist_ok=True)
File "<frozen os>", line 225, in makedirs

FileNotFoundError: [Errno 2] No such file or directory: ''
```

The fix here is to fallback to `os.curdir` on an empty string.