fix(file-router): preserve empty children for layout routes (#2572) (CP: main)

[platosha]

Fixes #2571

(cherry picked from commit c9189dfaa2ec6b832b1e15413b38baaf47312bf3)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.81%. Comparing base (466bd69) to head (4203a5b).

Files	Patch %	Lines
...ile-router/src/vite-plugin/createRoutesFromMeta.ts	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2585 +/- ## ======================================= Coverage 90.81% 90.81% ======================================= Files 66 66 Lines 2765 2765 Branches 528 528 ======================================= Hits 2511 2511 Misses 79 79 Partials 175 175 ``` | [Flag](https://app.codecov.io/gh/vaadin/hilla/pull/2585/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/2585/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | `90.81% <0.00%> (ø)` | | 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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[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