R1:1644ef185ecf

Fix system agent toggling in MySQL strict mode

Summary: `''` is not a valid integer.

Test Plan: Used `bin/accountadmin` to turn bot flag on and off for a user.

Reviewers: btrahan, Firehed

Reviewed By: Firehed

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9046