rgw: don't error out on empty owner when setting acls

Fixes: #6892
Backport: dumpling, emperor
s3cmd specifies empty owner field when trying to set acls on object
/ bucket. We errored out as it didn't match the current owner name, but
with this change we ignore it.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>