Generate empty delta as {} instead of null

A delta between two documents that have no changes is currently producing a value of null, but should produce a delta value of {}

This is causing issues for CBL recieving a delta for an empty change (CBL-833)

The fix for this is special handling specific to unchanged documents/roots, so take care when changing, because the main algorithm is recursive - unchanged objects that are nested should still be omitted from the delta. Maybe read a `null` value and transform it at a higher level for the root.