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>

fs/bcachefs/super-io.c

@@ -1312,7 +1312,10 @@ void bch2_sb_to_text(struct printbuf *out, struct bch_sb *sb,
 	prt_printf(out, "Device index:\t%u\n", sb->dev_idx);
 
 	prt_printf(out, "Label:\t");
-	prt_printf(out, "%.*s", (int) sizeof(sb->label), sb->label);
+	if (!strlen(sb->label))
+		prt_printf(out, "(none)");
+	else
+		prt_printf(out, "%.*s", (int) sizeof(sb->label), sb->label);
 	prt_newline(out);
 
 	prt_printf(out, "Version:\t");