txpool: don't validate block transactions if the pool is empty (#12973)

* txpool: don't validate block transactions if the pool is empty

Fix shall prevent from wasting the CPU during the major sync. Block
transaction don't need to be re-validated when the txpool is empty.

Fixes: #12903

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>