fix TypeError: cannot concatenate 'str' and 'NoneType' objects

LEARNER-5369

lms/templates/dashboard/_dashboard_course_listing.html

@@ -48,7 +48,7 @@ from util.course import get_link_for_about_page, get_encoded_course_sharing_utm_
     %>
     <%
         mode_class = course_verified_certs.get('display_mode', '')
-        if mode_class != '':
+        if mode_class:
             mode_class = ' ' + mode_class ;
     %>
   % else: