fix(authenticator): enforce required validation for empty phone number field

[VarshithaPamisetty]

Description

A required phone number field (SignUpFormField.phoneNumber(required: true)) still accepted an empty value. formatPhoneNumber prepended the dial code to empty input ("" -> "+1"), so the value looked non-empty, passed validation, and the form submitted a bogus dial-code-only number instead of showing the required error. This leaves empty input untouched so the required/empty check runs.

Screenshots

Before	After

Before: an empty required phone number passed validation and reached the SignUp call. After: it is blocked client-side with "Phone Number field must not be blank."

Testing

• Added a widget test: a required phone field submitted empty now shows "Phone Number field must not be blank." (fails without the change).
• authenticator suite green: sign_up_form_test, form_field_controller_test, and 284 golden UI tests pass; dart analyze and dart format clean.
• Verified in the example app: empty required phone is now blocked client-side instead of reaching the SignUp call.

Fixes #4079

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.12%. Comparing base (a6ec724) to head (7bc4551).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7296      +/-   ##
==========================================
+ Coverage   40.98%   41.12%   +0.14%     
==========================================
  Files         121      121              
  Lines        8273     8274       +1     
  Branches     3598     3599       +1     
==========================================
+ Hits         3391     3403      +12     
+ Misses       4882     4871      -11     

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