null tenant id in MD model causes nullpoint exception

https://github.com/opendaylight/neutron/blob/master/neutron-spi/src/main/java/org/opendaylight/neutron/spi/NeutronObject.java#L34

above line seems we support null tenant id into neutron MD model, but when it is passed backed into SPI model, we will run into null point failure

at https://github.com/opendaylight/neutron/blob/master/neutron-spi/src/main/java/org/opendaylight/neutron/spi/NeutronObject.java#L49