CTP-3560 avoid PHPUnit errors 3

Author: watson8

classes/grade_judge.php

@@ -65,13 +65,15 @@ public function get_grade_capped_by_submission_time($submission) {
     }
 
     /**
-     * @param $grade
+     * @param int|float $grade
      * @return float
      */
     private function round_grade_decimals($grade) {
-        if ($grade !== null) {
-            return round($grade, 2);
+        if ($grade === '' || $grade === null) {
+            // Avoid PHPUnit exception passing null or empty string to round().
+            return null;
         }
+        return round($grade, 2);
     }
 
     /**