rgw: check object name after rebuilding it in S3 POST

Fixes: #6088
Backport: bobtail, cuttlefish, dumpling

When posting an object it is possible to provide a key
name that refers to the original filename, however we
need to verify that in the end we don't end up with an
empty object name.

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>