`alertId` in `TransactionEvent` can be `null`

[edag94]

AlertId returns null for an AlertEvent from this tx hash: 0x91b64b7935b98366381c33a519655b3d769f0a7b054011828387fe6ca08fbb98

[edag94]

It's possible that AlertId isn't an optional property, in which case the property should be

{
  // ...
  alertId: string | null
}