fix(typings): correctly extract nested oprional form group array typing

[ntziolis]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/reactive-forms/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

This PR addresses an edge case the previous PR #180 did not cover.

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Assuming the current Formtype:

export interface FormType {
  items: {
    tags?: FormControl<string[] | null>
  }[]
}

When using ValuesOf<ControlsOf<FormType>>, tags has the type string[] | null instead of string[] | null | undefined.

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[ntziolis]

Apologies, this edge case slipped through in the last PR