fix: check for null tx

[cazala]

The following code throws many times:

const tx = await getTransaction(txId)

if (!isPending(tx) && tx.recepeit) {

Because when we get a transaction like this:

const tx = await getTransaction(txId)

The value of tx can be null because:

export async function getTransaction(txId: string): Promise<{ recepeit: TxReceipt } & TxStatus> {
  const [tx, recepeit] = await Promise.all([
    eth.wallet.getTransactionStatus(txId),
    eth.wallet.getTransactionReceipt(txId)
  ])

  return tx ? { ...tx, recepeit } : null
}

And then when we do:

!isPending(tx) && tx.recepeit

isPending returns null:

export function isPending(tx) {
  return tx && (tx.blockNumber === null || tx.status === TRANSACTION_STATUS.pending)
}

Then !isPending(tx) evaluates to true and the following expression evaluated is tx.recepeit which throws Cannot read property 'recepeit' of null 💥💥💥

This PR fixes that.

Also, is it okay that the property is called recepeit and not receipt ? I searched it and it's like that all over the library...