[Enhancement] - Return immediately if no transaction

[shenkeyao]

Return Ok([]) if there's no transaction instead of blocking.

May need to update how we handle responses on the HotShot side first before/instead of here.

Zulip discussion starts from here: https://espresso.zulipchat.com/#narrow/channel/461578-environment-mainnet/topic/Builder.20timeouts/near/485724870.