bcachefs: show none if label is not set

If label is not set, the Label tag in superblock info show '(none)'.

```
[Before]
Device index:                               0
Label:
Version:                                    1.4: member_seq

[After]
Device index:                               0
Label:                                      (none)
Version:                                    1.4: member_seq
```

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>