rspec:feature-flags failing at null_hypothesis feature flag

Failed job: https://gitlab.com/gitlab-org/gitlab/-/jobs/988140281

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These feature flags appears to be UNUSED
If they are really no longer needed REMOVE their .yml definition
If they are needed you need to ENSURE that their usage is covered with specs to continue.
- null_hypothesis
Feature flag usage check failed.
==> 'bundle exec scripts/used-feature-flags' failed (1) in 1 seconds.

Looks related to the changes in !52268 (diffs) which replaced

stub_feature_flags(null_hypothesis: false)

with

stub_experiments(null_hypothesis: :candidate)

and without calling

Feature.enabled?(:null_hypothesis)