fix: update the student list progress bar behavior

- Progress bar now displays "No Interaction" when a project has no
  recorded interactions (task list is null) instead of "NaN%"
- Progress bar is now grey (not_started) instead of red (fail) when a project
  has no recorded interactions

src/app/units/states/students-list/students-list.tpl.html

@@ -121,9 +121,9 @@ <h4>No students found</h4>
             <progress class="task-progress" animate="true">
               <bar
                 ng-repeat="bar in student.task_stats"
-                value="bar.value"
+                value="(bar.key === 'fail') && (bar.value !== bar.value) ? 0 : bar.value"
                 type="{{statusClass(bar.key)}}">
-                <span ng-hide="bar.value < 10">{{bar.value}}%</span>
+                <span ng-hide="bar.value < 10">{{bar.value !== bar.value ? 'No Interaction' : (bar.value + '%')}}</span>
               </bar>
             </progress>
           </td><!--/progress-column-->