interfacedef's method() generates strange signature for a nullable return type

> I want to specify an interface with a method that has a nullable return type.
> ``` kotlin
> var IMyInterface = interfacedef("IMyInterface") {
> method("get", PredefinedType.string.nullable)
> }
> ```
>
> But it generates to:
> ``` kotlin
> interface IMyInterface
> {
> fun get() : StringNullable
> }
> ```
>
> I expected `fun get() : String?`

_Originally posted by @al-darmonski in https://github.com/JetBrains/rd/issues/206#issuecomment-877036647_