internal: fast exit if getTxReceipt notfound

[jsvisa]

In the eth backend, the GetTransaction will always return nil error, but the tx maybe is null, so we don't have to continue the following logic if tx notfound

go-ethereum/eth/api_backend.go

Lines 316 to 319 in 4b06e4f

	func (b *EthAPIBackend) GetTransaction(ctx context.Context, txHash common.Hash) (*types.Transaction, common.Hash, uint64, uint64, error) {
	tx, blockHash, blockNumber, index := rawdb.ReadTransaction(b.eth.ChainDb(), txHash)
	return tx, blockHash, blockNumber, index, nil
	}

[holiman]

LGTM
For other reviewers: the tx being nil might indicate that it is a transaction in the pool, but for obtaining the receipt, digging the tx out from the pool doesn't solve it, so returning nil here seems fine