Handle compute node records with no timestamp.

Commit 94560ab57d9fc23673f42017e6f2a78cb2b66b7a added a fast-path
to avoid work when baremetal compute nodes haven't changed, but if
the compute message has None for its updated-at timestamp (which
can happen during startup) the comparison crashes.

Fixes bug: 1096719
Change-Id: Ib8add431d975eee133deefea62eb53479a97396e
(cherry picked from commit fad69df25ffcea2a44cbf3ef636a68863a2d64d9)