Follow-up from "Improve tests' speed and de-quarantine tests"

The following discussion from !14320 (merged) should be addressed:

• @rymai started a discussion:

  Let's fix that in a follow-up docs-only MR:

  Then, let's initialize an instance variable for labels to allow an empty array as default value when such information is not passed during the resource fabrication, since this optional. [Between the attributes and the `fabricate!` method](https://gitlab.com/gitlab-org/gitlab-ee/blob/1a1f1408728f19b2aa15887cd20bddab7e70c8bd/qa/qa/resource/issue.rb#L18), add the following: