Fix bison "empty" warnings
Summary: See https://www.gnu.org/software/bison/manual/html_node/Empty-Rules.html. Depends on D13970.
Test Plan: Less warnings.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D13971