toggleClass expects a boolean, not a truthy value for 2nd param

Previously we were passing in undefined, which was fine until you toggled
true, then wanted to go back.
see: http://api.jquery.com/toggleclass/#toggleClass-className-switch

fixes CNVS-5365

test plan:
- have a user with a submitted assignment that hasnt been graded
- in GradeBook, open the message students who...
- select "scored less than"
- select "Haven't submitted yet"
- select "Haven't been graded yet"
	- without fix, null will be displayed by the user
	- with the fix, everything will look normal

Change-Id: Ic05b8c2eec513ef26f8f3cdf26d8b328c7a850e1
Reviewed-on: https://gerrit.instructure.com/37206
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

Author: Brad Horrocks

public/javascripts/message_students.js

@@ -131,7 +131,8 @@ define([
         $message_students_dialog.find("[name=subject]").val(currentSettings.subjectCallback(option.text, cutoff));
       }
       $message_students_dialog.find(".cutoff_holder").showIf(option.cutoff);
-      $message_students_dialog.find(".student_list").toggleClass('show_score', option.cutoff || option.score);
+
+      $message_students_dialog.find(".student_list").toggleClass('show_score', !!(option.cutoff || option.score));
       $message_students_dialog.find("button").attr('disabled', student_ids.length == 0);
 
       var student_ids_hash = {};