restapi: Prevent NPE when getting statistics

VDSM doesn't report statistics for all of the network
interfaces (i.e. vlans, see [1] for more details)
therefore the restapi shouldn't expect a wrapper not
to be null and prevent autoboxing to avoid NPE.

[1] https://bugzilla.redhat.com/675560

Change-Id: Iab3422ef11f35b7a75d39f62cc5caa6f827ac7f4
Bug-Url: https://bugzilla.redhat.com/993123
Signed-off-by: Moti Asayag <masayag@redhat.com>