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
[GEOS-5800] GeoServer won't parse relative external graphic locations in SE 1.1 styles
Valid xml
Merge pull request #251 from Gnafu/master

Typo in documentation
[GEOS-5769] JDK 7 gives a preference to the pure java PNG writer even if the native one is available
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.
Revert "GEOS-5844 fix nulls in SecureCatalog.list"

This reverts commit b076629ebce1dc4cbdfe2eff622cc361ad0ebc8d.
[GEOS-5854] Add file browser support for GDAL formats
Clean up the kml feature access code and separate it from kmlutils, remove all kmlutils dead methods
GEOS-5849 added localized rule labels support to GetLegendGraphic output
Add support for extended data and schemas in kml
Adding a WFS KML output format
implementing list with vararg sort by, for now only handling single argument
GEOS-5847 fixed issue with nested layergroups
Ensure network links are always getting downloaded, without shrinking the bbox
Merge pull request #253 from mbarto/geos5849_internationalized_getlegendgraphic

GEOS-5849 added localized rule labels support to GetLegendGraphic output
Merge pull request #250 from ischneider/GEOS-5844

GEOS-5844 fix nulls in SecureCatalog.list
GEOS-5851 Make sure we don't synch up computing a non cacheable feature type each time we need a feature source
Fix build
GEOS-5852 default filter chain does not create HTTP session even with session creation enabled