BUGFIX: Enforce unique names also for root children

[nezaniel]

Until now, we checked uniqueness for node names for an explicitly given origin of the parent. While this makes sense in general, the constraint checks fails for more than zero dimensions when the parent is a root node, because root nodes still have a zero dimensional origin which then is not properly checked.

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Checklist

• [x] Code follows the PSR-2 coding style
• [x] Tests have been created, run and adjusted as needed
• [x] The PR is created against the lowest maintained branch
• [ ] Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• [ ] Reviewer - The first section explains the change briefly for change-logs
• [ ] Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions