azureblob/b2/dropbox/gcs/koofr/qingstor/s3: fix 0 length files

In 0386d22cc9 we introduced a test for 0 length files read the
way mount does.

This test failed on these backends which we fix up here.

backend/googlecloudstorage/googlecloudstorage.go

@@ -966,6 +966,7 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 	if err != nil {
 		return nil, err
 	}
+	fs.FixRangeOption(options, o.bytes)
 	fs.OpenOptionAddHTTPHeaders(req.Header, options)
 	var res *http.Response
 	err = o.fs.pacer.Call(func() (bool, error) {