fix: Don't include null traits in transient identifier

[khvn26]

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Changes

Fixes #4597.

This clarifies typing and discards null traits from the transient identifier generation.

How did you test this code?

Added a null trait to corresponding integration test.

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Sep 9, 2024 9:02am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Sep 9, 2024 9:02am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Sep 9, 2024 9:02am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4598	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4598	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4598	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4598	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4598	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4598	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-55983 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.15%. Comparing base (b2a1899) to head (c848da6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4598   +/-   ##
=======================================
  Coverage   97.15%   97.15%           
=======================================
  Files        1157     1157           
  Lines       40023    40023           
=======================================
  Hits        38886    38886           
  Misses       1137     1137           

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