[3.1.1 backport] Panic inside Mutable1xBody for nil shallow copy

We don't have sufficient handling for a nil doc body retruned by shallow copy inside the function Mutable1xBody.

When we take a shallow copy of the document here https://github.com/couchbase/sync_gateway/blob/fe16b931b3923052ae6a8d4e6b400300ec86b6ab/db/revision_cache_interface.go#L160

If we get a nil body returned here we can get a panic here https://github.com/couchbase/sync_gateway/blob/fe16b931b3923052ae6a8d4e6b400300ec86b6ab/db/revision_cache_interface.go#L165 by trying to assign to a nil map. We should have a check to see if b in this code is nil and if so we should initialise an empty body so we can assign the rev and doc id without panicing.