MDL-35435: Fix invalid CSV file when assignment has no enrolled student and submission in offline grading worksheet

mod/assign/gradingtable.php

@@ -302,6 +302,10 @@ class assign_grading_table extends table_sql implements renderable {
             }
         }
 
+        // When there is no data we still want the column headers printed in the csv file.
+        if ($this->is_downloading()) {
+            $this->start_output();
+        }
     }
 
     /**