Check for null before running 'fetch a modulepreload script graph' optional steps

[dandclark]

Currently fetch a modulepreload module script graph optionally proceeds to steps to fetch the descendants of the preloaded module without checking the result of fetch a single module script for failure. It doesn't make sense to try to fetch descendants if the original fetch failed (e.g. due to 404), and fetch the descendants of and link doesn't expect to be passed a null module script.

Thus, this change guards the optional steps with a null check.

This clarification should be a no-op for implementers, who must already have this sort of check in place; there's not much that they could do otherwise.

[dandclark]

I'm splitting this bugfix/clarification off from #5658 per @littledan 's suggestion.

[littledan]

This is a nice improvement for clarity.

[domenic]

Great, thank you!