do not include empty aliases in the compressed scopes

[wandersoncferreira]

Previously to this PR an empty entry in the scopes alias dictionary would be added to the compressed scopes result:

(is (= #{"+admin" "+baz" "x" "+foo"}
         (sut/scopes-compress #{"foo" "bar" "baz" "x"}
                              {"+admin" #{"foo" "bar"}
                               "+baz" #{"baz"}
                               "+foo" #{}})))

An additional check in scopes compression fixed it.

[yogsototh]

LGTM!