GEOS-5844 fix nulls in SecureCatalog.list

a catalog filter can potentially result in nulls being in the
returned iterator. an example is a LocalWorkspaceCatalogFilter
when a WMS virtual service getcaps request occurs.

forward port from 5edc2cc67de9aa922c798c1e8049939178a9d529

had to add means to override check on GeoServerSecurityManager
so test cases would not depend on full spring context