tremplin: Fix a possible crash during shutdown

If we shut down before LXD starts then server.lxd is nil and we crash.
Add a check for that case.

BUG=None
TEST=CQ

Change-Id: I51cbf15a037df651b0ed938d83d6a53067f13edd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/tremplin/+/2588970
Tested-by: David Munro <davidmunro@google.com>
Auto-Submit: David Munro <davidmunro@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
Reviewed-by: Fergus Dall <sidereal@google.com>