Support null/undefined values in array returned from LinksFunction

[markdalgleish]

I'd primarily like this to make it easier to deal with the conditional nature of cssBundleHref when using CSS Modules, Vanilla Extract, etc.

When adding a link descriptor referencing cssBundleHref, the value could be undefined in the case where a CSS bundle isn't generated, so you need to write it this way to make TypeScript happy:

import { cssBundleHref } from '@remix-run/css-bundle';

export const links: LinksFunction = () => {
  return [
    ...(cssBundleHref ? [{ rel: "stylesheet", href: cssBundleHref }] : [])
  ];
};

It'd be nice to be able to avoid a noisy array spread and supply an explicit null value instead:

export const links: LinksFunction = () => {
  return [
    cssBundleHref ? { rel: "stylesheet", href: cssBundleHref } : null,
  ];
};

We could potentially support undefined too, in which case you could write this:

export const links: LinksFunction = () => {
  return [
    cssBundleHref && { rel: "stylesheet", href: cssBundleHref }
  ];
};