When UpdateStatus was not present, the empty values of the timestamps
would be present:
"UpdateStatus": {
"StartedAt": "0001-01-01T00:00:00Z",
"CompletedAt": "0001-01-01T00:00:00Z"
}
To fix this, make the timestamps pointers, so they can be set to nil
when they should not be shown.
Also make UpdateStatus itself a pointer, so an empty object does not
show up when there is no UpdateStatus.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>