ENYO-2935: zero-copy return the file-map when no destination is given

This give the opportunity to the caller to `_realize()` locally (current)
behaviour or to use the file map as a scatter-gather array to build an
archive of files (eg. the `multipart/form-data` used by Ares `genZip`).

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>