MB-10378: stat vbucket-details should return correct value

When requesting the stat vbucket-details with a vbucket the
server doesn't have, it should return a not_my_vbucket instead
of an empty reply. This commit will reutrn not_my_vbucket error
in this case.

Change-Id: I84076156ffca361ba5fec9b54dec0a9bfcae8859
Reviewed-on: http://review.couchbase.org/34443
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>