Add filename to combined source map if needed

[dummdidumm]

When source maps are combined and the leading map is empty, sources is not set. Add the filename to the empty array in this case. This may be an upstream issue (but maybe it's also intended, let's see) but either way for our cases it's necessary to set sources even for empty maps.
They were not added before this change so I adjusted the tests. The js.map output now is closer to what it would be like if there was no preprocessing and there's no JS - in that case Svelte outputs an empty map with sources set which looks like this

{
    version: 3,
    names: [],
    sources: [ '..path/to/$layout.svelte' ],
    sourcesContent: [ '<p>asd</p>\r\n' ],
    mappings: ';;;;;;;'
}

Related sveltejs/kit#424

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• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[dummdidumm]

Now that this is unblocked in Kit we could just wait with this PR until the upstream issue is answered/resolved

[benmccann]

Do we still want this PR now that things are working in SvelteKit?

[dummdidumm]

There might be a need for it: sveltejs/svelte-loader#171 (comment) . Needs more investigation if that would fix it though.

[Conduitry]

This has been released in 3.40.2.