use an EmptyStringAsNullAdapter in NeutronObject re. null tenant_id

use an EmptyStringAsNullAdapter in NeutronObject re. null tenant_id

perhaps in JAXB it's better to place such special empty/null logic in
such an adapter instead of directly into the getter.. at worst, it
shouldn't hurt.  See also some public posts re. this in general, e.g.
https://stackoverflow.com/questions/10869748/jaxb-return-null-instead-empty-string

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: I47ec8270ba42e787d21adff51d89052c4442f2b7
Signed-off-by: Michael Vorburger <vorburger@redhat.com>