Enforce root layout container to be always present

[Wumpf]

What

Previously, it was possible to end up without a root blueprint container which can cause weird ui behavior like not being able to add space views.
This is now solved by making up a root container upon reading the blueprint layout from the data store.

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Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: rerun.io/viewer
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Recent benchmark results
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[Wumpf]

On a previous iteration with this some examples stopped working according to notes. Need to check if that's still the case

[Wumpf]

On a previous iteration with this some examples stopped working according to notes

still the case with web based examples at the very least

[abey79]

Very nice cleanup!