migrations: add NOT NULL constraint to versions.commit_time

Change-Id: If38eb3cdfebd1ea960280830a64a61fc8db5d6f7
Reviewed-on: https://team-review.git.corp.google.com/c/424249
Reviewed-by: Channing Kimble-Brown <ckimblebrown@google.com>

migrations/000009_versions_commit_time_not_null.down.sql

@@ -0,0 +1,5 @@
+-- Copyright 2009 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+ALTER TABLE versions ALTER COLUMN commit_time DROP NOT NULL;

migrations/000009_versions_commit_time_not_null.up.sql

@@ -0,0 +1,5 @@
+-- Copyright 2009 The Go Authors. All rights reserved.
+-- Use of this source code is governed by a BSD-style
+-- license that can be found in the LICENSE file.
+
+ALTER TABLE versions ALTER COLUMN commit_time SET NOT NULL;