migrations: make licenses.module_path and licenses.version nullable

Drop the NOT NULL constraint on licenses.module_path and
licenses.version, so that we can stop inserting into these
rows in the next CL.

In a later CL, licenses.module_path and licenses.version will be
dropped.

For golang/go#39629

Change-Id: Ie917498306d46e03a761af32406356774a18fc78
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/272086
Trust: Julie Qiu <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: julieqiu

migrations/000049_drop_licenses_module_path_version_not_null.down.sql

@@ -0,0 +1,11 @@
+-- Copyright 2020 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.
+
+BEGIN;
+
+ALTER TABLE licenses
+    ALTER COLUMN module_path SET NOT NULL,
+    ALTER COLUMN version SET NOT NULL;
+
+END;

migrations/000049_drop_licenses_module_path_version_not_null.up.sql

@@ -0,0 +1,11 @@
+-- Copyright 2020 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.
+
+BEGIN;
+
+ALTER TABLE licenses
+    ALTER COLUMN module_path DROP NOT NULL,
+    ALTER COLUMN version DROP NOT NULL;
+
+END;