Fix Authz API cache returning empty identities

When entering the if block the user variable (set at the top of the function) is shadowed by the one declared inside the if block. After leaving the scope of the if block, user is empty again. This leads to the function(s) returning ("", nil) (i.e. no error, but also no result), and subsequently the AuthZ API complains about the empty ownerID in the payload for creating the application registration.

See details in https://gitlab.cern.ch/webservices/webframeworks-planning/-/issues/1365