Stop writing optional theme fields

[lhecker]

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Other settings model classes call JsonUtils::SetValueForKey with the private _##value member as the value. Since _##value is an optional, this prevented writing out unset, optional fields. The new Themes class deviated from this and this commit brings it back in line with the others.

Closes #13544

Validation Steps Performed

• Create a { "name": "test" } theme
• Save settings via the UI
• Optional/Defaulted fields aren't written ✅

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:tada:Windows Terminal Preview v1.16.252 has been released which incorporates this pull request.:tada:

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