Align command validation and currentValues nullish handling

[woksin]

Summary

Fixes two command correctness issues in Arc React forms and dialogs:

• Empty string values were rejected by core required-property validation even when no explicit empty-string validator existed, making client validation stricter than server/domain validation.
• Reactive currentValues / useCommand syncing ignored explicit null and undefined values, so cleared external state could leave stale command values for validation and submit.

Fixed

• Fixed command required-property validation to treat only null and undefined as missing, letting explicit validators decide whether empty strings are invalid. (Align command validation and currentValues nullish handling #2307)
• Fixed currentValues and useCommand value syncing so explicit null and undefined values clear command properties instead of leaving stale values. (Align command validation and currentValues nullish handling #2307)

Changed

• Clarified CommandForm validation docs to distinguish required field metadata from explicit empty-string validation rules. (Align command validation and currentValues nullish handling #2307)

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.43.7