[11.x] Fix SyntaxError on Vite prefetch with empty assets

[jnoordsij]

When $assets is empty and the 'aggressive' strategy is applied in the Vite prefetcher, the resulting JS includes the fragment:

        const fragment = new DocumentFragment
        [].forEach((asset) => fragment.append(makeLink(asset)))
        document.head.append(fragment)

When executed this yields an error Uncaught SyntaxError: expected expression, got ']'. Although not hurtful (there is nothing to do anyways), it is still a slight nuisance.

This MR fixes the issue by adding a semicolon to eagerly terminate the previous statement. An alternative way would be to rewrite the entire statement to not write any html at all when $assets is empty, given that there is nothing to do.