ENYO-2291: fix PhoneGap build, broken by new `Ares.Model.Project`

- Updated `appId` is to be stored in the `Ares.Model.Project` only if
  non-empty.  Otherwise an existing `appId` is lost over an
  un-successful build attempt.

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