Forkchoice start using proper root (#6062)

* Skip update head if justiifed root is zero hashes

* Start with genesis root instead

Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>

beacon-chain/blockchain/head.go

@@ -42,7 +42,13 @@ func (s *Service) updateHead(ctx context.Context, balances []uint64) error {
 	// Get head from the fork choice service.
 	f := s.finalizedCheckpt
 	j := s.justifiedCheckpt
-	headRoot, err := s.forkChoiceStore.Head(ctx, j.Epoch, bytesutil.ToBytes32(j.Root), balances, f.Epoch)
+	// To get head before the first justified epoch, the fork choice will start with genesis root
+	// instead of zero hashes.
+	headStartRoot := bytesutil.ToBytes32(j.Root)
+	if headStartRoot == params.BeaconConfig().ZeroHash {
+		headStartRoot = s.genesisRoot
+	}
+	headRoot, err := s.forkChoiceStore.Head(ctx, j.Epoch, headStartRoot, balances, f.Epoch)
 	if err != nil {
 		return err
 	}