[resultdb] Sleep between intervals if there's no invocation tasks to run.
[resultdb] Sleep between intervals if there's no invocation tasks to run.

Change-Id: Ib35989150eddca789ec9cc98d6d0aad75b98d13b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2003938
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go