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

[platosha]

Fixes #2571

[sonarcloud[bot]]

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 24.4 #2572 +/- ## ======================================= Coverage ? 95.01% ======================================= Files ? 66 Lines ? 4557 Branches ? 664 ======================================= Hits ? 4330 Misses ? 182 Partials ? 45 ``` | [Flag](https://app.codecov.io/gh/vaadin/hilla/pull/2572/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/vaadin/hilla/pull/2572/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | `95.01% <100.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin#carryforward-flags-in-the-pull-request-comment) to find out more.

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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 c9189dfaa2ec6b832b1e15413b38baaf47312bf3 error: could not apply c9189dfa... 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".