level-monitor: Fix issue with warp syncing (#2053)

When warp syncing a node we import a header of the parachain around the
tip of the chain. This header is currently not imported as finalized
block (should be fixed at some point as well), the parent headers are
not yet present (still being synced) and thus, we run into a panic. Even
if there is a case where a leaf could not be found in the database, this
probably means that the db is broken and it will fail somewhere elese.