Merge pull request #2464 from edx/dcs/default-css-class

The default course CSS class should be empty string rather than None.

common/lib/xmodule/xmodule/course_module.py

@@ -364,7 +364,7 @@ class CourseFields(object):
     # way to add in course-specific styling. There needs to be a discussion
     # about the right way to do this, but arjun will address this ASAP. Also
     # note that the courseware template needs to change when this is removed.
-    css_class = String(help="DO NOT USE THIS", scope=Scope.settings)
+    css_class = String(help="DO NOT USE THIS", scope=Scope.settings, default="")
 
     # TODO: This is a quick kludge to allow CS50 (and other courses) to
     # specify their own discussion forums as external links by specifying a