Backup and restore should be less restrictive of contents of stack-settings.yml

[thomas-anderson-bsl]

Currently if stack-settings.yml is empty, or the backups: key is empty, then you cannot do a backup (expected) or a restore (bad).

This should be less restrictive. If stack-settings.yml doesn't exist, is empty, or backups: is empty, then backups should not be possible. However, a restore should always be possible, regardless of those preconditions.