bcachefs: Fix infinite loop in journal_entry_btree_keys_to_text()

Fix an infinite loop when bkey_i->k.u64s is 0.

This only happens in userspace, where 'bcachefs list_journal' can print
the entire contents of the journal, and non-dirty entries aren't
validated.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>