[connman-qt] Always return a network service object as the default route.

Always return a valid object, representing an invalid network service,
when no default route is set. Simplifies the API as no default route
set was represented as either a null network service or a valid network
service object with the state property set as an empty string.
Merge pull request #98 from amccarthy/master

[connman-qt] Always return a network service object as the default route...