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/s3/s3.go

@@ -1765,6 +1765,7 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 		Bucket: &o.fs.bucket,
 		Key:    &key,
 	}
+	fs.FixRangeOption(options, o.bytes)
 	for _, option := range options {
 		switch option.(type) {
 		case *fs.RangeOption, *fs.SeekOption: