fix(file-router): enable server fallback with client layout and empty client routes (#3280) (CP: 24.6)

[vaadin-bot]

No description provided.

[sonarqubecloud]

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 SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (24.6@965cfc1). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...e-router/src/runtime/RouterConfigurationBuilder.ts	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             24.6    #3281   +/-   ##
=======================================
  Coverage        ?   92.66%           
=======================================
  Files           ?       85           
  Lines           ?     3161           
  Branches        ?      772           
=======================================
  Hits            ?     2929           
  Misses          ?      182           
  Partials        ?       50           

Flag	Coverage Δ
unittests	92.66% <83.33%> (?)

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