Mechanism to pass equivalent of no optional argument explicitly

Say you have a decorator with two optional string parameters:

`extern dec test(target: unknown, arg1?: string, arg2?: string);`

How do you pass arg2 without passing arg1? `@test(null, "hello")` doesn't compile and shouldn't compile and even if it did, it would be observably different to the JS implementation than the `undefined` value it gets when the optional args are not passed at all. Do we need `undefined` in TypeSpec? I have long hoped we didn't but this seems like a gap.