Fix flaky test on pipeline metrics

What does this MR do?

Fix flaky test as a result of a memoized nil value. #340022 (closed)

How to setup and validate locally (strongly suggested)

Run bundle exec rspec spec/lib/gitlab/ci/pipeline/metrics_spec.rb spec/services/ci/create_pipeline_service/custom_yaml_tags_spec.rb.

Before: spec/services/ci/create_pipeline_service/custom_yaml_tags_spec.rb fails After: spec/services/ci/create_pipeline_service/custom_yaml_tags_spec.rb passes

Does this MR meet the acceptance criteria?

Conformity

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Security

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