Pack rule operator exists / nexists should not have criteria pattern mandatory

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!

Trace pack installation and criteria validation for the exists and nexists rule operators, using the reproduction snippets in the issue as the starting case. Verify how other operators validate pattern values. Done means a pack containing an exists or nexists criterion without pattern installs successfully, while invalid criteria remain rejected.