FIX #606, correct URL for categories if Blog is on /home

[lerni]

Description

Get correct URL for categories if Blog is on /home. For categories $this->Blog()->Link() returns just / but should return /home.

Manual testing steps

Create a Blog on /home and see categories fail

[User Warning] Request handler SilverStripe\CMS\Controllers\ModelAsController does not have a url_segment defined. Relying on this link may be an application error
GET /category/...

Line 573 in /var/www/html/vendor/silverstripe/framework/src/Control/RequestHandler.php

Issues

• BlogObject filter URLs break if the Blog is the homepage #606

Pull request checklist

• The target branch is correct
  • See picking the right version
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• The commit messages follow our commit message guidelines
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• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[GuySartorelli]

Please also target this PR to the 4.3 branch so it can be released as a patch instead of waiting for the next minor release

[GuySartorelli]

Please also update the commit message both to reflect the bug that is being fixed and to include the correct commit prefix (there's a link in the PR checklist for commit messages) - and check the checklist while you're at it. That checklist is there to help you as the contributor make sure you've got all these sorts of things correct and it's ready for review.

[GuySartorelli]

Finally, if you could add a unit test for this that would be great!