fix(router): Fix relative link generation from empty path components

[atscott]

Partial resubmit of #26243
Fixes incorrect url tree generation for empty path components with children.
Adds a test to demonstrate the failure of createUrlTree for those routes.
Fixes #13011
Fixes #35687

[atscott]

There are two reproductions (https://stackblitz.com/edit/angular-lazy-route-relative-bug, https://stackblitz.com/edit/relative-link-eager-loading) in #35687 that aren't fixed by the change as it is now. However, both of those would work if the fix were changed to
return new Position(segmentGroup, false, 0);
instead of
return new Position(segmentGroup, segmentGroup.segments.length === 0, 0);

Adding cleanup, blocked, and presubmit label to indicate that I need to investigate if this fix can be extended to include #35687, adding a unit test to cover that situation, and running presubmits to ensure it doesn't break anything.

Edit: test for this case would be:

  it('should support pathless child of pathless root', () => {
    // i.e. routes = {path: '', loadChildren: () => import('child')...}
    // forChild: {path: '', component: Comp}
    const p = serializer.parse('');
    const empty = new UrlSegmentGroup([], {});
    p.root.children[PRIMARY_OUTLET] = empty;
    empty.parent = p.root;
    const t = create(empty, -1, p, ['lazy']);
    expect(serializer.serialize(t)).toEqual('/lazy');
  });

presubmit has some failures though

[atscott]

global presubmit looks good

[atscott]

global presubmit with additional change (processChildren = segmentGroup === tree.root instead of segmentGroup.segments.length === 0) to address #35687.

Some things to note for review:

• All three added tests fail without the change
• The generated links without the change are completely wrong (/a/(b) and //(lazy)) meaning this shouldn't be behavior anyone could have been relying on and this is purely a bug fix
• Global presubmit shows no failures

[AndrewKushnir]

LGTM 👍 Thanks for the fix @atscott!

[atscott]

merge assistance: I'm the router owner so pullapprove won't be happy without global approval

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