Modifier with only optional args resolves into `never` when bound with modifier keyword

[mogstad]

When using the modifier keyword, and binding an all optional modifier the resolved type isn’t invokable.

Repro:

type Signature = {
  Args: {
    Named: {
      value?: string;
    };
  };
};

declare let Modifier: ModifierLike<Signature>;

<template>
  {{#let (modifier Modifier value='anything') as |aModifier|}}
    <div {{aModifier}}></div>
  {{/let}}
</template>

aModifier resolves to Invokable<(named: PrebindArgs<HTMLDivElement, never>) => ModifierReturn> when the argument isn’t option the type is: Invokable<(element: Element, named?: PrebindArgs<{ value: string; }, "value"> | undefined) => ModifierReturn>