Feature request: "isNotZero" helper

### Which packages are impacted by your issue?

@graphprotocol/graph-ts

### Describe the feature request

We often need to perform an "is not zero check", e.g.

```typescript
event.transaction.value.isZero() === false
```

It would be quite handy if there were an `isNotZero` helper provided natively by `@graphprotocol/graph-ts`.
