prevent_updates(): allow changing value from NULL

[vkrasnovyd]

In short: revert removing this check: d1e0fe4#diff-5690774bb27aa9eb24e7abfe4bb33e9d59dab68fcccda1685a9df26f8a63f657L434-L437

Usage: when the reading side of the 1:1 relation should have constant: true. Example: meeting_poll_default.used_as_*_poll_config_in_meeting_id.