Config migration script bug fix

Summary: i think way back in D10490 I didn't incorporate feedback correctly. make this code right as it fatals in this codepath as is. Fixes T6508.

Test Plan: @joshuaspence to the rescue (I remain unable to test this effectively with my baby-clean installation.)

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley, joshuaspence

Maniphest Tasks: T6508

Differential Revision: https://secure.phabricator.com/D10833