Allow input arguments using Record to have default value

Now if we use data_type for input arguments such as 

```python
Graph([
  Root([
    Link("create", TypeRef["CreateInput"], link_fn)
  ])
], data_types={
  "CreateInput": Record[{"id": Integer, "name": Optional[String]}]
})
```

We can not set default value for a `id` or `name` fields of a `CreateInput` data_type 