Fix a bad lookup in hovercards

Summary:
If you try to pull the hovercard of something you can no longer see (maybe you loaded the page, then the policy changed) there won't be a value in the array here.

(The rest of the code anticipates this possibility.)

Test Plan: Hovered some stuff.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14987