[ASC] Optional parameters should be undefined over null

[AuHau]

Currently, if generate for example this WIDL:

interface{
  storeChunk(data: bytes, stamp: string?): string
}

The generated code for the call has stamp: string | null, which makes it a practically required parameter in AssemblyScript. It would make sense with AssemblyScript/TypeScript semantic instead of null use undefined so the parameter could be truly optional.