MDL-48861 assign: Make the requires grading filter consistent

Consistently include submissions with a null grade (see MDL-47694).

mod/assign/locallib.php

@@ -1521,7 +1521,7 @@ class assign {
                         s.assignment = :assignid AND
                         s.timemodified IS NOT NULL AND
                         s.status = :submitted AND
-                        (s.timemodified >= g.timemodified OR g.timemodified IS NULL)';
+                        (s.timemodified >= g.timemodified OR g.timemodified IS NULL OR g.grade IS NULL)';
 
         return $DB->count_records_sql($sql, $params);
     }