R1:af1fef242c66

Fix an issue with editing pre-space objects using a form with no visibility controls

Summary:
WMF ran into this after their update. Here's the setup:

- When you enable Spaces, we leave all existing objects set to `null`, which means "these belong to the default space". This is so we don't have to go update a trillion objects.
- New objects get set to the default space explicitly (`PHID-SPCE-...`) but older ones stay with `null`.
- If you edit an older object (like a task) from the time before…