TypeScript Resolvers plugin doesn't allow null to be returned from a promise

**Describe the bug**
Given a query with an optional response:

```
type Query {
subscription: SubscriptionDetails
}
```

I should be able to write a resolver that returns a promise that returns null:

```
const subscription: QueryResolvers["subscription"] = async (
_root,
_args,
) => {
return await fetchResult(); // Might return null
}
```

However, I can't because the return type is defined like this:

`Maybe>`

So I can return null _immediately_ (thanks to the `Maybe`), but if I return a Promise then I have to return `ResolverTypeWrapper`, which is defined as:

`export type ResolverTypeWrapper = Promise | T;`

Where the type T itself (SubscriptionDetails in the example) isn't null, so I can't return null.

**Proposed Solution**:

If the response is optional then the generated code should be something like:

`Maybe>>`