fix: generate correct SSR code for the case where pending is an attribute

[paoloricciuti]

Closes #16886

There were two problems here: the first is that when pending is an attribute we weren't checking if the fragment contained async...this means if you did use await in a boundary with a pending attribute it was not creating the $$render.async wrapper.

However, that showcased another problem in that generated code:

import 'svelte/internal/flags/async';
import * as $ from 'svelte/internal/server';

function pending($$renderer) {}

export default function App($$renderer) {
	if (pending) {
		$$renderer.push(`<!--[!-->${pending($$renderer)}<!--]-->`);
	} else {
		$$renderer.push(`<!--[-->`);

		{
			const x = (await $.save(Promise.resolve()))();
		}

		$$renderer.push(`<!--]-->`);
	}
}

if pending is truthy instead of just invoking it (since it will push it's content to the $$renderer) we were also pushing to the template the return value (which is undefined since it's not returning anything.

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