Follow-up from !30458: commands should return array by default

The following discussion from !30458 (merged) should be addressed:

• @dzaporozhets started a discussion:

  Its not like I have better idea but I want to say I was confused with this line. To understand why it's to_a on second argument and why we are ok with doing nil.to_a, I need to take a look at delete_knative_services, delete_knative_istio_metrics and DeleteCommand implementation.

  Maybe a better solution for the future is to convert all kubectl methods to return empty array instead of nil if there is no commands to execute. For example:

  def delete_knative_istio_metrics
    if cluster.application_prometheus_available?  
      ["kubectl delete --ignore-not-found -f #{METRICS_CONFIG}"] 
    else
      []
    end
  end

  If you feel the same way you can create a ~"technical debt" for that. It would not block this MR as you followed existing style of code.