pubsub: fix panic on Topic.ResumePublish (#2444)
pubsub: fix panic on Topic.ResumePublish (#2444)

When Topic.ResumePublish is called before Topic.Publish,
the underlying scheduler is not initialized yet, leading to a panic.

This patch makes sure scheduler.Resume is only called
when the scheduler is not nil.

Co-authored-by: Alex Hong <a9ccb06cbb47e15d85bcb2cc9161076d89dbbe0a@users.noreply.github.com>
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go