Stop spamming block config export with meaningless empty configuration when a visibility group has not been added

Reminder to try to figure out how other modules prevent this unneeded config.

Possibly related to 🐛 Visibility groups don't remove existing visibility rules Active

I’ve seen other contrib modules somehow clean up their config, and I’m curious to learn how this is done.

I think it's worth a) adding an update script that cleans up existing block configurations, b) make sure that BVG doesn't add elements to configurations when it isn't needed.

@damienmckenna didn't you solve this in 🐛 Empty settings added to all block configurations Fixed . We can re-scope this issue to the clean-up update hook task, if that covers this issue?