Use empty collections to ensure the prefix directories are available in
kopia no matter what user data we find.
This helps ensure we have some set of things we can assume about
non-failed backups including:
They have a valid snapshot ID for data
They have a valid snapshot ID for backup details
Eventually, this will allow us to say that backups using a base that
doesn't have a prefix directory is invalid in some way
SharePoint tests are disabled because lists and pages ignore the none
target in selectors
---
#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🐛 Bugfix
#### Issue(s)
* #2550
#### Test Plan
- [x] ⚡ Unit test
- [x] 💚 E2E