don't show empty list if there is no error data

lms/templates/courseware/instructor_dashboard.html

@@ -110,7 +110,11 @@ table.stat_table td {
       <ul>
         % for (summary, err) in course_errors:
         <li>${summary | h}
+        % if err:
           <ul><li><pre>${err | h}</pre></li></ul>
+        % else:
+          <p>&nbsp;</p>
+        % endif
         </li>
         % endfor
         </ul>