feat: make default value optional for ValueSignal

[taefi]

Description

Fixes #2746

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[codecov]

Codecov Report

Attention: Patch coverage is 97.61905% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.84%. Comparing base (6de563d) to head (7f9f5f4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ts/generator-plugin-signals/src/SignalProcessor.ts	97.61%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3160      +/-   ##
==========================================
+ Coverage   92.65%   92.84%   +0.19%     
==========================================
  Files          85       85              
  Lines        3187     3328     +141     
  Branches      778      800      +22     
==========================================
+ Hits         2953     3090     +137     
- Misses        182      186       +4     
  Partials       52       52              

Flag	Coverage Δ
unittests	92.84% <97.61%> (+0.19%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.8% Duplication on New Code

See analysis details on SonarQube Cloud