Add a null check for the username field value

[DilshanSenarath]

Purpose

$subject

Related Issue

• Username field is auto poppulated with the subject claim from Fed Auth during JIT with consented username and password flow product-is#21578

Related PR

• Add a config to autofill the username with subject attribute while jit provisioning carbon-identity-framework#6225

[wso2-jenkins-bot]

🦋 Changeset detected

The changes in this PR will be included in the next version bump.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.32%. Comparing base (332755b) to head (1664078).
Report is 347 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7223   +/-   ##
=======================================
  Coverage   32.32%   32.32%           
=======================================
  Files          42       42           
  Lines         897      897           
  Branches      221      204   -17     
=======================================
  Hits          290      290           
  Misses        607      607           

Flag	Coverage Δ
@wso2is/core	32.32% <ø> (ø)

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