[BridgeJS] Optionals with custom types

Optionals are supported like so:
```swift
@JSFunction func foo() throws(JSException) -> String? // ✅
```
but this does not work with objects:
```swift
@JSClass struct Foo {
    @JSGetter var id: String
}
@JSFunction func foo() throws(JSException) -> Foo? // ❌
```
