bcachefs: skip mount option handle for empty string.

The options parse in get_tree will split the options buffer, it will
get the empty string for last one by strsep(). After commit
ea0eeb89b1d5 ("bcachefs: reject unknown mount options") is merged,
unknown mount options is not allowed (here is empty string), and this
causes this errors. This can be reproduced just by the following steps:

    bcachefs format /dev/loop
    mount -t bcachefs -o metadata_target=loop1 /dev/loop1 /mnt/bcachefs/

Fixes: ea0eeb89b1d5 ("bcachefs: reject unknown mount options")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>