text: Title: Pack rule operator exists / nexists should not have criteria pattern mandatory
  Body: ## SUMMARY
  
  When using exists / nexists rule operator, we need to provide a criteria pattern which is not correct, as the value of this pattern needs to be null / should not be there.
  
  ### STACKSTORM VERSION
  
  st2 3.7.0, on Python 3.6.8
  
  ##### OS, environment, install method
  
  Running on Docker in Mackbook Pro (x86)
  
  ## Steps to reproduce the problem
  
  Use exists / nexists rule operator in criteria section. Omit the value of criteria pattern (as it is not needed for this operator).
  ```
  criteria:
    trigger.org_id:
      type: exists
  ```
  ## Expected Results
  
  The pack should get installed correctly as all information is available.
  
  ## Actual Results
  
  Pack installation fails and we need to add a pattern field unnecessarily.
  ```
  criteria:
    trigger.org_id:
      type: exists
      pattern: "placeholder"
  ```
  
  Making sure to follow these steps will guarantee the quickest resolution possible.
  
  Thanks!
  