Regex in workflow validator for multi-line text custom field does not work as expected

Issue Summary

If a multi-line text field is cleared it will prevent issues from being transitioned if there is a regex validator on the transition

Steps to Reproduce

1. Create a field of type multi-line text.
2. Add a workflow validator
3. Choose "Regular Expression check"
4. Choose a Multi line text field and in the regular expression type .*
5. Click on Add
6. Publish the workflow
7. Create an issue
8. Set the multi-line text field to a value
9. Unset the multi-line text
10. Perform the transition

Expected Results

Should be able to transition the issue even though the field is empty

Actual Results

Workaround

Currently there is no known workaround for this behaviour.