Default value should be empty dict literal string

[caesar2164]

• student view uses empty dict if actual value is missing

[caesar2164]

this preempts a possible bug as mentioned here: https://github.com/Stanford-Online/xblock-in-video-quiz/pull/8

[coveralls]

Changes Unknown when pulling 9c07439c6831ecc99f6f4ce828dd10ee04711090 on giulio/preempt-empty-dict-bug into on master.

[stvstnfrd]

Pleeease don't squash PRs mid-review; the "remove P tag" should have been a separate commit.

[stvstnfrd]

+1