Fix matrix widget to warn on blank matrices

Test Plan:
1. Add a matrix widget with an answer in the Editor
2. Click score with an empty matrix in the question
3. Observe that we show "Make sure you fill in all cells in the matrix." instead scoring as incorrect (score as empty)

Reviewers: aria

Reviewed By: aria

Subscribers: alex

Differential Revision: http://phabricator.khanacademy.org/D13524