Improve validation of gauge temporal validity The current version of a gauge should always have a validity represented by a non-empty time range. Thus, do not accept empty time ranges on input. Nevertheless, continue to accept storing empty time ranges (no CHECK constraint), because a new version with equal start date but longer validity might still cause the previous version to get an empty time range. In passing, add a missing NOT NULL constraint: if both start and end date are omitted, the range will be unbounded, but it should never be NULL.