1. Add a new config variable profilesforenrolledusersonly = true (by default)
2. When showing the profile page, if a courseid isn't defined (this is the public view) AND the user is not enrolled in any courses AND profilesforenrolledusersonly = true then replace the profile description with something like "This profile description will not be shown until this person is enrolled in at least one course".
3. When editing the profile page, if the description is blank AND the user is not enrolled in any courses AND profilesforenrolledusersonly = true then completely hide the description field from the editing page.