cls/cmpomap: empty values are 0 in U64 comparisons

cls/cmpomap: empty values are 0 in U64 comparisons

previously, when trying to use cmpomap interfaces on an omap key with
an empty value, U64 comparisons would fail to decode with -EIO. so
cmp_set_vals() and cmp_rm_keys() are unable to update or remove such
keys

for backward-compatibility with rgw's data sync error repo, where the
keys used to have empty values, enable these comparisons by treating an
empty value as 0

Fixes: https://tracker.ceph.com/issues/52128
Signed-off-by: Casey Bodley <cbodley@redhat.com>