In Rules 7.x-2.2, checking if a user exists is possible with no custom coding. See the Rules maintainer's explanation
To check if a user exists, use the following:
- Condition: "Entity exists by property
- Entity Type: "User"
- Property: "Name" (or any other user field);
- Value: "instructor name" (based on your related post; otherwise this would be the ).
- Negate this condition. (so that the rule will not fire if a user exists)
This can be done either in one reaction rule or as an e.g. "condition" component. If using the component, it's vital to pass the node object to the component (in component settings) as a variable so that "instructor name" is available in the component.
As I noted in your related question that you will almost always want a prior condition that ensures the content type in question contains is of type "course". Otherwise, this rule will run when you either may not want it to or it should not run.