More control over maybeValue

(This was originally requested in #1935, but was closed as a duplicate of #1885 which was incorrect. Hear me out.)

Since a GraphQL optional field may either be missing entirely or explicitly set to `null`, input types benefit from `maybeValue: T | null | undefined`. With that input type it is easy to implement deletion of a field by checking for `null` explicitly in the mutation resolver.

However, now all optional _output_ types can also be `null` or `undefined`. In my codebase, we exclusively use `undefined` as our 'not there' value, which causes some trouble with conversions between types when returning results from resolvers or writing re-usable functions, since the output types are not 100% compatible. This causes `as XXX` usage, which then sometimes hides other problems.

SO...

If I can generate with config like:
```yaml
...
config:
maybeValue:
inputValue: T | null | undefined
others: T | undefined
```

It'll make the generated types match closer with (my) reality.

In case you're wondering... Using the default `maybeValue: T | null` hides the fact that undefined are still possible on inputs specifically, especially with the default use of optionals.

I case it's not 100 clear, I would like to achieve this:
- input types: `T | null | undefined` for things that are optional in the input type
- output types: `T | undefined` for things that are optional in the output type

Can this be done?

EDIT: Why not `null` as the optional output type? Because if it is returned as a field, additional more-specific resolvers won't be called, since `null` is valid in that case. Accidentally returning `{name: 'cat', kittens: null}` will cause the `Cat.kittens` resolver to not be called, since a valid value is already present, although this may be a bug in the server.