util/resolver: ignore invalid (empty) scope

[morpheu]

This PR solves a problem specific for OCI registry.

When starting auth process hitting /v2 route, registry returns an empty scope which leads to a subsequent request as follow:

$ curl -v https://sa-saopaulo-1.ocir.io/v2/ 2>&1 | grep -i www
< WWW-Authenticate: Bearer realm="https://sa-saopaulo-1.ocir.io/20180419/docker/token",service="sa-saopaulo-1.ocir.io",scope=""

And them when hitting url https://sa-saopaulo-1.ocir.io/20180419/docker/token, buildkit try to send a invalid query string scope=::

This PR ignores empty scopes