This PR was squashed before being merged into the 4.1 branch (closes#27472).Discussion----------[DI] Ignore missing tree root nodes on validate| Q             | A| ------------- | ---| Branch?       | 4.1| Bug fix?      | yes| New feature?  | technically yes| BC breaks?    | no     <!-- seehttps://symfony.com/bc -->| Deprecations? | no| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->| Fixed tickets |#27450| License       | MIT| Doc PR        | symfony/symfony-docs#... <!-- required for new features --><!--Write a short README entry for your feature/bugfix here (replace this comment block.)This will help people understand your PR and can be used as a start of the Doc PR.Additionally: - Bug fixes must be submitted against the lowest branch where they apply   (lowest branches are regularly merged to upper ones so they get the fixes too). - Features and deprecations must be submitted against the master branch.-->Commits-------b3cdfc6 [DI] Ignore missing tree root nodes on validate

…abbuh)This PR was merged into the 4.2-dev branch.Discussion----------[Config] deprecate tree builders without root nodes| Q             | A| ------------- | ---| Branch?       | master| Bug fix?      | no| New feature?  | yes| BC breaks?    | no| Deprecations? | yes| Tests pass?   | yes| Fixed tickets || License       | MIT| Doc PR        |While reviewing#27472 I wondered if we really need support config trees without a root node. If we did not support it, users wouldn't create pseudo configuration classes when they were actually not needed.Commits-------c2ce153 deprecate tree builders without root nodes