add a validator to copyright holder

Author: fdiblen

copier/questions/essential.yml

@@ -46,4 +46,7 @@ copyright_holder:
   type: str
   placeholder: Netherlands eScience Center
   help: Who is the copyright holder?
-  when: "{{ template_profile != 'minimum' and AddPackageDetails }}"
+  validator: >-
+        {% if not copyright_holder %}
+        This field cannot be empty
+        {% endif %}