test: cover empty security requirements

[kriptoburak]

Summary

• Add high-level SecurityRequirement coverage for empty requirement objects.
• Assert the anonymous requirement marker survives promotion and rendering.

Validation

• go test ./datamodel/high/base ./datamodel/low/base ./what-changed/model

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.75%. Comparing base (d4c45ce) to head (ac6761f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #601   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         281      281           
  Lines       34061    34096   +35     
=======================================
+ Hits        33977    34012   +35     
  Misses         51       51           
  Partials       33       33           

Flag	Coverage Δ
unittests	99.75% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[daveshanley]

LGTM.