Delete of document with body {} doesn't result in CBS tombstone

If a document with body `{}` is deleted under shared bucket access, it doesn't result in a Couchbase Server tombstone.  The empty body comparison here doesn't consider the previous body as non-empty, and so doesn't trigger the body delete subdoc operation.  

The document is a Sync Gateway tombstone, and so will be replicated properly as a delete, but the document won't be purged from the server based on the metadata purge interval. 

Needs review of the motivation for the original change from length zero to length two - presumably there are scenarios where Sync Gateway is converting documents with empty bodies to {} during read, and this was to handle that scenario.  May require a more significant refactor to otherwise mark empty bodies on read.