restapi : Handle empty int values in JAX-B parser

JAX-B replacement IntegerParser should handle empty
values for integer fields. Set the default value to 0.

Change-Id: I3823d8894f316d78f753034fdbd89a79f480516e
Bug-Url: https://bugzilla.redhat.com/1064456
Signed-off-by: Ravi Nori <rnori@redhat.com>