Legalpad - fix a bug for documents with no signatures

Summary: be sure to use array() (and not null) so we don't fatal if we have no signatures

Test Plan: no more fatal when editing a signature-less document

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8000