Prioritize using user.username whenever provided

[wlee221]

Description of changes

This properly handles username input on confirmSignUp whenever usernameAlias is set to phone_number.

Problem
Before confirmPassword is submitted, we stringify phone_number using composePhoneNumberInput(this.phoneNumber). The problem occurs when a valid username has already been passed through the user prop. In that case, we disable the form for username and just use the provided user.username. But we still try to compose phone number from this.phoneNumber, which is undefined, so we get an error message for that.

This PR fixes it by prioritizing username to be user.username and only look at form values whenever it's not the case.

Issue #, if available

Fixes #7267

Description of how you validated changes

Since username validations has been problematic especially with custom usernameAlias, I'll do testing across multiple auth components.

• Case 1: usernameAlias = 'phone_number': testing with gist
• Case 2: usernameAlias = 'email': testing with gist

Checklist

• PR description included
• yarn test passes

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[thaddmt]

Is this file meant to be included / are the prettier changes meant to be included in this pr?

[wlee221]

Good catch, this isn't supposed to be here. I have added that to suppress prettier while I'm working on this, but I created #8136 to better address this.

[codecov-commenter]

Codecov Report

Merging #8050 (967f303) into main (3881d53) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #8050   +/-   ##
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  Coverage   77.66%   77.66%           
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  Files         236      236           
  Lines       16505    16505           
  Branches     3535     3535           
=======================================
  Hits        12819    12819           
  Misses       3575     3575           
  Partials      111      111           

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