Do not set ETag header in initial stored, lazy rendition "empty" response

Prior to the fix for NXP-27337 in Nuxeo 10.10-HF15, the initial response for a stored, lazy, Folderish rendition included the following headers and and did NOT include an ETag header:

Content-Disposition: inline; filename=inprogress
Content-Type: text/plain; empty=true
Content-Length: 0

The fix for NXP-27337 introduced an ETag header (with the MD5sum of the empty string) into the initial response which differs from the Apache Chemistry CMIS Reference Implementation and confuses some CMIS clients.

The pre-10.10 HF15 functionality regarding the ETag response header in this scenario should be restored.