fix(file-router): preserve empty children for layout routes

[platosha]

Fixes #2571

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             24.4    #2572   +/-   ##
=======================================
  Coverage        ?   95.01%           
=======================================
  Files           ?       66           
  Lines           ?     4557           
  Branches        ?      664           
=======================================
  Hits            ?     4330           
  Misses          ?      182           
  Partials        ?       45           

Flag	Coverage Δ
unittests	95.01% <100.00%> (?)

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[vaadin-bot]

Hi @platosha and @Lodin, when i performed cherry-pick to this commit to main, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick c9189df
error: could not apply c9189df... fix(file-router): preserve empty children for layout routes (#2572)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".