Make build_status.timestamp nullable

The Guix Build Coordinator doesn't record timestamps for events
currently. This is something to fix, but allow for build statuses without
timestamps as they're not necessary.

sqitch/verify/build_status_nullable_timestamp.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:build_status_nullable_timestamp on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;