Make user_id optional for track_login_failure

[y9v]

Fixes #3840.

What does this PR do?
This PR fixes a bug when Datadog::Kit::AppSec::Events.track_login_failure is raising an ArgumentError when user_id is not provided and user_exists flag is set to false.

Motivation:
Issue #3840

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.83%. Comparing base (46c8c8b) to head (44834a4).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3841   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files        1264     1264           
  Lines       75678    75682    +4     
  Branches     3720     3720           
=======================================
+ Hits        74039    74045    +6     
+ Misses       1639     1637    -2     

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-08-12 15:09:08

Comparing candidate commit 44834a4 in PR branch fix-track-login-failure with baseline commit 46c8c8b in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 22 metrics, 2 unstable metrics.

scenario:hold / resume

• 🟩 throughput [+50457.227op/s; +52408.229op/s] or [+2.453%; +2.548%]

[y9v]

/merge

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