rgw: Avoid segfault when OPA authz is enabled

rgw: Avoid segfault when OPA authz is enabled

When `rgw_use_opa_authz=true`, radosgw would segfault on any request that
didn't target a specific object or bucket, because `s->object` or `s->bucket`
would be `nullptr` in that case, but that code path would try to dereference
them anyway.

This commit only adds the `object_name`, `subuser`, `user_info` and
`bucket_info` JSON objects if the corresponding `s->X` object is defined,
thereby avoiding segfaults in radosgw when Open Policy Agent authorization is
enabled.

Fixes: https://tracker.ceph.com/issues/55286
Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>