fix: map null value to std::nullopt for optional types

[camchis]

Fixes #1184

When a prop on a fabric view is removed it's value returns null rather than undefined, this causes issues on optional types in Nitro where only undefined and explicitly specified values are accepted

These changes allow an optional to accept a null value which will be mapped to std::nullopt

If an optional has null specified as an accepted type then this will not be affected by these changes and you will still have distinction between null and undefined values if needed

[vercel]

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