MDL-57255 gradereport_user: Fix default value for graderaw

graderaw is a PARAM_FLOAT, the default to empty string was incorrect

grade/report/user/lib.php

@@ -544,7 +544,7 @@ private function fill_table_recursive(&$element) {
                 }
 
                 if ($this->showgrade) {
-                    $gradeitemdata['graderaw'] = '';
+                    $gradeitemdata['graderaw'] = null;
                     $gradeitemdata['gradehiddenbydate'] = false;
                     $gradeitemdata['gradeneedsupdate'] = $grade_grade->grade_item->needsupdate;
                     $gradeitemdata['gradeishidden'] = $grade_grade->is_hidden();