all: cleanup the APIs for initializing genesis (#25473)

* all: polish tests

* core: apply feedback from Guillaume

* core: fix comment

cmd/devp2p/internal/ethtest/chain.go

@@ -139,7 +139,7 @@ func loadChain(chainfile string, genesis string) (*Chain, error) {
 	if err != nil {
 		return nil, err
 	}
-	gblock := gen.ToBlock(nil)
+	gblock := gen.ToBlock()
 
 	blocks, err := blocksFromFile(chainfile, gblock)
 	if err != nil {