edx/edx-platform — pull request #5893 by waheedahmed (2014-11-10 13:45:20+00:00)
Title: Fixed null answer in checkbox problem as incorrect.

Ticket: [TNL-765] (https://openedx.atlassian.net/browse/TNL-765)

If a checkbox group problem have no correct answer in it and student submit response without selecting any checkbox then he gets a green checkbox but no credit for that problem. Fixed by marking null answer as incorrect.
Added a unit test.