fix JAXB usage/design bug in NeutronObject re. null tenant_id

fix JAXB usage/design bug in NeutronObject re. null tenant_id

@XmlElement on a (package visible..) field and then special logic in the
getter was perhaps an unholy combination.  I can't quite explain why it
worked most of the time but not always.  One would think that the tests
going through the getter should also execute that special logic, but
alas, clearly in some of the test runs (and perhaps in production...)
they did not, see the recurring NEUTRON-159 problem.

background: https://bugs.opendaylight.org/show_bug.cgi?id=4775 (and its
old https://git.opendaylight.org/gerrit/#/c/31324 and
https://git.opendaylight.org/gerrit/#/c/31361/)

JIRA: NEUTRON-159
Change-Id: I986dc6f4bea2f08becea21936bedd831643d212e
Signed-off-by: Michael Vorburger <vorburger@redhat.com>