[FOSS] Fix tests for usage_data:generate_ci_template_events

What does this MR do and why?

Fixes the tests on FOSS for the rake task usage_data:generate_ci_template_events. EE-only templates can expand to nil on FOSS and should not have events generated.

As pointed out by @smcgivern in #377698 (comment 1133335120)

It seems like Verify/Browser-Performance.gitlab-ci.yml gets expanded to nil. I'm not sure if https://gitlab.com/gitlab-org/gitlab/-/blob/5c0da4eefe8470642798241ce01049a2295b8daa/lib/gitlab/template/gitlab_ci_yml_template.rb#L42-44 is related.

This is because browser performance testing is a premium feature (the excludes are removed in EE).

Screenshots or screen recordings

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How to set up and validate locally

Run the test in FOSS mode

FOSS_ONLY=1 bin/rspec spec/tasks/gitlab/usage_data_rake_spec.rb:72

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