fix printing of null-valued (i.e., optional or empty list) relations

scope4m/src/main/jastadd/ASTPrinting.jadd

@@ -14,7 +14,8 @@ aspect Debugging {
           } else {
             name += ":= ";
           }
-          result += "|--" + name + method.invoke(this).toString().split("\\r?\\n")[0];
+          Object value = method.invoke(this);
+          result += "|--" + name + (value == null ? "<null>" : value.toString().split("\\r?\\n")[0]);
         } catch (IllegalAccessException e) {
           e.printStackTrace();
         } catch (java.lang.reflect.InvocationTargetException e) {