migrations: set not-null constraint on version_map.go_mod_path

version_map.go_mod_path should always be set. In events when it is
unknown (such as for status 40x requests), it will be set to the empty
string. A not-null constraint is now set on this column.

For golang/go#36811
For golang/go#37002

Change-Id: I9c5e01fbe9d772cb4cb7e43394aac2564a494734
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/242437
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000023_change_version_map_go_mod_path_not_null.down.sql

@@ -0,0 +1,9 @@
+-- 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 version_map ALTER COLUMN go_mod_path DROP NOT NULL;
+
+END;

migrations/000023_change_version_map_go_mod_path_not_null.up.sql

@@ -0,0 +1,9 @@
+-- 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 version_map ALTER COLUMN go_mod_path SET NOT NULL DEFAULT '';
+
+END;