TransactionReceipt.effectiveGasPrice type

[Velenir]

Describe the bug
A clear and concise description of what the bug is.
There's no effectiveGasPrice on receipts for some networks, for example BSC. But it is required on TransactionReceipt type, which can lead to errors in runtime that Typescript won't catch at compile time.

Reproduction Steps

1. POST to BSC endpoint

curl --location --request POST 'https://bsc-dataseed.binance.org/' \
--header 'Content-Type: application/json' \
--data-raw '{
	"jsonrpc":"2.0",
	"method":"eth_getTransactionReceipt",
	"params":[
		"0x527d0c4afe45466a7e2beabfc84b55e02b22297b12e20404af94909f279625d0"
	],
	"id":1
}'

2. No effectiveGasPrice

I'm not sure if it's an idiosyncrasy of BSC or also present on other networks

Marking it optional may be enough of a fix

[ricmoo]

In v5, types are not checked against strictNullCheck, so all returns are assumed to be able to return null If they return anything.

In v6, strictNullCheck is enabled, so the type will include null. :)