Cannot generate null value for type without mocking parent

Assuming the following schema:

```
type Query {
fields: [Field!]!
}

interface Field {
id: ID!
}

type FooField implements Field {
sub: SubFoo
}

type SubFoo {
name: String!
}
```

and I have a query as follows:

```
query ExampleQuery {
fields {
...myFooFragment
}
}

fragment myFooFragment on FooField {
sub {
name
}
}
```

It's valid for this query to return `null` for the `sub` key.

When I mock that in a test as follows:

```
environment.mock.queueOperationResolver((operation) =>
MockPayloadGenerator.generate(operation, {
SubFoo: () => null,
})
);
```

I would expect this to set the `sub` key to null in the response. What I'm seeing, though, is a generated value:

```
{ sub: { name: '' } }
```

If I instead do this, it works...

```
environment.mock.queueOperationResolver((operation) =>
MockPayloadGenerator.generate(operation, {
FooField: () => ({ sub: null }),
})
);
```

I get the expected value:

```
{ sub: null }

```

But since not every key will have a nested key to null out, this isn't a general solution.

I think this line is in error; instead of using a nullish coalescing operator, it should be explicitly testing for undefined so that `null` values are accepted from the generator functions. https://github.com/facebook/relay/blame/98162d36249cbdb162155427c0d721192de78d81/packages/relay-test-utils/RelayMockPayloadGenerator.js#L712

Start in packages/relay-test-utils/RelayMockPayloadGenerator.js at the referenced line around 712 and trace how generated type values are selected. Run the relevant relay-test-utils tests, then add coverage for a generator returning null and confirm the generated SubFoo field is null rather than populated.

Written by the indexing model from the issue text.