Error on lesson detailed statistics page when empty string entered for numerical question

When someone enters an empty string for a numerical question in a lesson, the detailed statistics page for that lesson throws an exception since the value is not a float.

Steps to reproduce:

1. Login as teacher.
2. Create lesson.
3. Add numerical question.
4. Login as student.
5. Take lesson answering the question with an empty string.
6. Switch back to teacher.
7. View Lesson -> Reports -> Detailed statistics.