Prevent empty array return for qa_tests

What does this MR do and why?

Make sure qa_tests do not return empty array. This can happen when stage_specs method doesn't find any of the stage specific specs which in turn will lead to setting KNAPSACK_TEST_FILE_PATTERN variable to {} which will lead to executing suites with no tests. Example: https://gitlab.com/gitlab-org/gitlab/-/jobs/5723239698

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