"None" was still appearing histogram data, causing js errors; fixes.

common/djangoapps/xmodule_modifiers.py

@@ -75,7 +75,7 @@ def grade_histogram(module_id):
 
     grades = list(cursor.fetchall())
     grades.sort(key=lambda x: x[0])          # Add ORDER BY to sql query?
-    if len(grades) == 1 and grades[0][0] is None:
+    if len(grades) >= 1 and grades[0][0] is None:
         return []
     return grades