Do not require new headers if lane is empty (#1725)
* do not require new headers if lane is empty * handle edge case (need proof-of-delivery-confirmations to be able to submit delivery tx) in required_source_header_at_target * clippy
* do not require new headers if lane is empty * handle edge case (need proof-of-delivery-confirmations to be able to submit delivery tx) in required_source_header_at_target * clippy