bcachefs: Ignore empty btree root journal entries

There's no reason to treat them as errors: just ignore them, and go with
a previous btree root if we had one.

Reported-by:  <syzbot+e22007d6acb9c87c2362@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>