ethclient: change Hash field of rpcBlock to pointer

[islishude]

It's null in a pending block

go-ethereum/internal/ethapi/api.go

Lines 500 to 504 in a82303f

	if number == rpc.PendingBlockNumber {
	// Pending blocks need to nil out a few fields
	for _, field := range []string{"hash", "nonce", "miner"} {
	response[field] = nil
	}