MB-7631 Interpret bp of 0 as empty body

By convention, in Couchstore/.couch files a body pointer of zero
indicates that the item has no body/has an empty body.

This commit makes CouchDB also have this behavior (otherwise, it will
attempt to read data from position 0 in the file when encountering a
deleted item)

Change-Id: If4229b68a2b6fc79484535619b7acfbc5055ca0e
Reviewed-on: http://review.couchbase.org/24586
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>