[Glint 2] Optional positional arguments in helpers are not respected

```gts
import Helper, { helper } from '@ember/component/helper';

class Foo extends Helper<{ Args: { Positional: [number?]; Named: { readonly str: string } }; Return: number }> {}
const bar = helper((_positional: [number?], _named: { readonly str: string }): number => 1);

<template>
  {{Foo str="123"}}
  {{bar str="123"}}
</template>
```

These two err but shouldn't.