fix: crash when user attribute is empty

[zhu-xiaowei]

Description

1. fix crash when user attribute is empty

General Checklist

• Added new tests to cover change, if needed
• Build succeeds using Swift Package Manager
• All unit tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• If breaking change, documentation/changelog update with migration instructions

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.85%. Comparing base (45a4ad4) to head (9b6d945).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
- Coverage   96.91%   96.85%   -0.06%     
==========================================
  Files          35       35              
  Lines        1685     1686       +1     
==========================================
  Hits         1633     1633              
- Misses         52       53       +1     

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