I read here that Jira Cloud does not support adding a workflow transition validator for a custom field but that I can get that functionality if I use Jira Suite Utlities (JSU).
So I installed JSU and went to edit my workflow but I don't see any additional validators when I edit a transition. I just see the original list of validators and the 'Field Required Validator' only shows Jira built-in fields.
I do see a single condition in the 'Add Condition to Transition' screen with '(JSU)' in the name: User Is In Any Users (JSU).