Send back empty votes + log in approval-voting in case candidate entry is missing. (#5925)

* Send back empty votes + log.

* Update node/core/approval-voting/src/lib.rs

Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>

Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>