feat(common): accept undefined inputs in NgTemplateOutlet

[rslawik]

Extend types of inputs to include undefined to avoid ?? null when using singals (e.g. viewChild).

Fixes #51225

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• 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?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Inputs don't accept undefined

Issue Number: #51225

What is the new behavior?

Accept undefined

Does this PR introduce a breaking change?

• Yes
• No

Other information

[mmalerba]

reviewed-for: public-api

[pkozlowski-opensource]

LGTM

Reviewed-for: fw-general
Reviewed-for: public-api

[devversion]

Might be worth a breaking change note when people are relying on the type of those public variables?

Reviewed-for: public-api

[JeanMeche]

@devversion You're making a good point. If we consider it a breaking change we won't be able to land it in v20.

Maybe let the TGP decide if we consider that breaking ?

[JeanMeche]

TGP is "Green" without having to cleanup g3.

I think we can consider this change non breaking.

[atscott]

This PR was merged into the repository by commit 82327f2.

The changes were merged into the following branches: main, 20.0.x

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