Improve assertions for code suggestion tests
For the following two tests, when there is an error and no choices
array returned the checkOutput
assertion fails with https://gitlab.com/gitlab-org/quality/performance/-/jobs/6907884425/viewer#L1470
This means that no successRate is recorded which makes it appear as if the tests pass.
https://gitlab.com/gitlab-org/quality/performance/-/blob/main/k6/tests/experimental/api_v4_code_suggestions_completions.js https://gitlab.com/gitlab-org/quality/performance/-/blob/main/k6/tests/experimental/api_v4_code_suggestions_generation.js
We should improve the assertion to fail gracefully in this case.