`[ID!]` type incorrectly treated as a scalar in `custom_queries.py`

An MRE can be found here: https://github.com/multimeric/AriadneCodegenBug.

In summary, a schema such as:

```graphql
type Query {
  boards(
    ids: [ID!]
  ): [String]
}
```

Incorrectly produces:
```python
class Query:
    @classmethod
    def boards(cls, *, ids: Optional[str] = None) -> GraphQLField:
        ...
```

`ids` should be `list[str]` or similar, and not a scalar `Optional[str]`.