If a student has just looked at a scored xmodule and not attempted it, their grade is `None`. This effectively means that this function almost always returns `[]` and so the histogram is not displayed. So I am not sure why these lines actually exist.

Another way to fix this is to make the first comparison `len(grades) == 1` but if the histogram is being calculated and only for problems with scores, we might as well always display it.

A sample query on prod:
+-------+--------------------------------------------+
| grade | COUNT(courseware_studentmodule.student_id) |
+-------+--------------------------------------------+
|  NULL |                                      6251 |
|    0 |                                        329 |
|    1 |                                        150 |
|    2 |                                      4502 |
+-------+--------------------------------------------+