fix(binder): make sure default value is read correctly

[cromoteca]

Fixes #3102, but needs tests.

[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 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.54%. Comparing base (3bb8032) to head (5a479ec).

Files with missing lines	Patch %	Lines
packages/ts/lit-form/src/BinderNode.ts	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3116      +/-   ##
==========================================
- Coverage   92.57%   92.54%   -0.04%     
==========================================
  Files          85       85              
  Lines        3164     3164              
  Branches      775      776       +1     
==========================================
- Hits         2929     2928       -1     
  Misses        183      183              
- Partials       52       53       +1     

Flag	Coverage Δ
unittests	92.54% <50.00%> (-0.04%)	⬇️

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