BTN-2095 allow metadata to be None when creating an SBD

An SBD can be created without an identifier and sent to the ODA where the sbd_id is fetched. It is at this point the metadata is created before storing in the ODA - the metadata represents the state of the entity in the ODA. So it should be possible to create an SBD python instance without metadata in the case that the SBD hasn't been persisted yet