Link to problem area:
https://github.com/matrix-org/matrix-doc/blob/1ad5ff4179cfa2b62d6be47ad6ed8bde95ec2e41/data/api/client-server/key_backup.yaml#L852-L880
Issue
This field is missing required: true like is done here.
The description says that if no keys are found, then empty (but not omitted) rooms property will be returned.
This whole schema should probably just be de-duplicated into the definitions folder like so.
