[embind] Fix TS definition for optional value object properties.

[brendandahl]

TS treats optional interface properties differently if they're defined with someProp?: type or someProp: type|undefined. When creating a object literal to match the interface, the ? doesn't require the property name whereas type|undefined requires the property name (though an undefined value is allowed).

Fixes #25978

[sbc100]

Can you explain what you mean by "doesn't require the field name"? Should the test include that case maybe?

[brendandahl]

interface FooOptional { 
    bar?: number | undefined
}
interface FooUndefined {
    bar: number | undefined
}

const foo: FooOptional = {} // OK
const foo: FooOptional = { bar: undefined } // OK
const foo: FooUndefined = {} // error, bar is required
const foo: FooUndefined = {bar: undefined} // OK

This new behavior is automatically tested since I added an optional field and I didn't have to change any of the other value_object tests in test/other/embind_tsgen_main.ts. Previously, they would have required the optionalInt field.

[sbc100]

I see, so when you say "it doesn't require the field name" you mean when creating an instance from a object literal? Maybe you could update the description to be more clear about?