[BUG] audiopolicy: engineconfigurable: prevent illegal map access

If any request to get OutputDevice for a given attributes is performed
before the cache has been initalized, an illegal access to the vector
happens.
This CL fixes by returning empty device vector when cache is empty.

Test: build & boot configurable engine target & basic audio

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: I1371765401441732319be56a9cae6fbf3c5cc2bf