Don't show `Next steps:` with no messages at all when plugin returns an
unitialized value of `HookMessage` (zero-initialization sets its type to
NextSteps and empty template).
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 296a6f5872)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>