Fix owner() policy check for networks

_remove_unauthorized_networks now covers the case when
tenant_id is None.

RM9736
Admin cannot delete in this scenario...

```python
def delete_network(context, id):
    ...
        net = db_api.network_find(context, id=id, scope=db_api.ONE)
        if not net:
            raise exceptions.NetworkNotFound(net_id=id)
        if context.tenant_id != net["tenant_id"]:
            raise exceptions.NotAuthorized()
        if net.ports:
            raise exceptions.NetworkInUse(net_id=id)
        ...
        db_api.network_delete(context, net)
```