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
Merge pull request #250 from ischneider/GEOS-5844

GEOS-5844 fix nulls in SecureCatalog.list