MDL-51720 assign: Always fallback on userid as a default sort

Even when the table has no userid column (this is normally the edit column).

mod/assign/gradingtable.php

@@ -406,8 +406,6 @@ class assign_grading_table extends table_sql implements renderable {
         foreach ($extrauserfields as $extrafield) {
              $this->column_class($extrafield, $extrafield);
         }
-        // We require at least one unique column for the sort.
-        $this->sortable(true, 'userid');
         $this->no_sorting('recordid');
         $this->no_sorting('finalgrade');
         $this->no_sorting('userid');
@@ -1364,6 +1362,16 @@ class assign_grading_table extends table_sql implements renderable {
                has_capability('moodle/grade:viewall', $context);
     }
 
+    /**
+     * Always return a valid sort - even if the userid column is missing.
+     * @return array column name => SORT_... constant.
+     */
+    public function get_sort_columns() {
+        $result = parent::get_sort_columns();
+        $result = array_merge($result, array('userid' => SORT_ASC));
+        return $result;
+    }
+
     /**
      * Override the table show_hide_link to not show for select column.
      *