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