Check params before clearing dataStoreCache (GEOS-6575)

See https://jira.codehaus.org/browse/GEOS-6575 for discussion.

Initial review here https://github.com/geoserver/geoserver/pull/668
Jody, all these casts should be superfluous, "class DataStoreCache extends CatalogResourceCache<String, DataAccess>", it should already return a DataAccess no?