make courseware/capa/util.py contextualize_text return gracefully if text = None

djangoapps/courseware/capa/util.py

 def contextualize_text(text, context): # private
     ''' Takes a string with variables. E.g. $a+$b. 
     Does a substitution of those variables from the context '''
+    if not text: return text
     for key in sorted(context, lambda x,y:cmp(len(y),len(x))):
         text=text.replace('$'+key, str(context[key]))
     return text