Should not mark empty item bodies as compressed

This results in CouchDB attempting to decompress nothing and blowing up:

initial call: xdc_vbucket_rep:init/1
pid: <0.17073.1>
registered_name: []
exception exit: badmatch,{error,corrupted_data,
[

{couch_compress,decompress,1}

,

{couch_doc,with_uncompressed_body,1}

,

{couch_doc,to_json_base64,1}

,

{xdc_vbucket_rep_worker,maybe_flush_docs,3}

,

{lists,foldl,3}

,

{xdc_vbucket_rep_worker,local_process_batch,5}

,

{xdc_vbucket_rep_worker,queue_fetch_loop,4}

]}
in function gen_server:terminate/6
ancestors: [<0.9075.0>,<0.9071.0>,xdc_replication_sup,ns_server_sup,
ns_server_cluster_sup,<0.60.0>]
messages: []
links: [<0.17403.1>,<0.17407.1>,<0.17410.1>,<0.17406.1>,<0.17401.1>,
<0.9075.0>]
dictionary: []
trap_exit: true