`flush` doesn't create or upload the file until the file is closed

fsspec version 2022.5.0
gcsfs version 2022.5.0

Code to reproduce:

import fsspec
fileobj = fsspec.open('gs://<insert-your-bucket-here>/test-write-flush', 'w', auto_mkdirs=True)
f = fileobj.fs.open(fileobj.path, mode=fileobj.mode)
f.write('w' * (2**20))  # is guaranteed to be larger than minimal block size
f.flush()  # does nothing visible - no file is created at the destination
f.close()  # now the file is created and has content

Upon debugging flush call, it seems that the check self.buffer.tell() < self.blocksize is always True, because the way things are implemented, self.buffer.tell() returns 0.
Furthermore, if I call manually what is in fsspec flush implementation after the check, meaning this code:

        if self.offset is None:
            # Initialize a multipart upload
            self.offset = 0
            try:
                self._initiate_upload()
            except:  # noqa: E722
                self.closed = True
                raise

        if self._upload_chunk(final=force) is not False:
            self.offset += self.buffer.seek(0, 2)
            self.buffer = io.BytesIO()

the file is still not created, although the underlying code in _upload_chunk does something.

Run the provided fsspec/gcsfs reproduction and trace the flush implementation, especially the buffer.tell() check, _initiate_upload(), and _upload_chunk(). Confirm the expected behavior by checking that flush creates or uploads the Google Cloud Storage object before close, while preserving the reported close behavior.