Confusing SSR error when let:foo has no corresponding <slot foo={...}>

[Rich-Harris]

Describe the bug

A component with slotted contents will fail to render if it has a let: directive but the corresponding <slot> has no properties.

To Reproduce

This REPL demo contains the bug, though it's not directly visible since it's SSR-only. The offending code is this:

	return `${$$slots.default ? $$slots.default() : ``}`;

$$slots.default corresponds to this:

	return `${validate_component(Foo, 'Foo').$$render($$result, {}, {}, {
		default: ({ id }) => `
		${validate_component(Bar, 'Bar').$$render($$result, { thing: things[id] }, {}, {})}
	`
	})}`

This fails with

TypeError: Cannot destructure property id of 'undefined' or 'null'.

Expected behavior

It might be nice if a warning was printed about the missing property. But the easy immediate fix is to always pass an object, i.e. $$slots.default({}). This way there's no disparity between <slot> and <slot someOtherProperty="whatever">.

Severity

Minor annoyance