./tools/clear-db fix

Summary:
Running ./tools/clear-db occasionally threw an error on trying to set eas_account_sync_key=None; it tried to do that because the default value for that column was returned $s None, which makes no sense
(since we set the server_default for that column to '0').

Both Eben and Spang ran into the error but I couldn't repro it until now - when the database is populated from the test dump,
or if it was created before the switch to using server_default (like the time of the test dump), the default values would be NULL.
This migration fixes that.

Test Plan: It's been tested on the test dump.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D98