Write a test for the null reference bug in HTML Injection check

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A quick fix for API Security: Testing failed: Object reference ... (#371740 - closed) was added in MR https://gitlab.com/gitlab-org/security-products/analyzers/api-fuzzing-src/-/merge_requests/837+. The quick fix didn't have any tests written for it. This is a follow-on issue to write tests for the bug that was resolved.