Don't set mustcapture for falsey values

Summary: See discussion in D3722. Good catch -- I'd definitely say this is a bug.

Test Plan: Rendered two <div />s, one with mustcapture set to true and one to false. The first got data-mustcapture; the second did not.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3723