WPS workspace specific configurations fails to load

Any WPS workspace specific configuration will fail to load with a NULL pointer exception.

The method initXStreamPersister of class WPSXStreamLoader does not invoke the super method super.initXStreamPersister(xp, gs); neither initiate properly some variables. The catalog variable is one of the variables that is not properly initiated. Without the catalog the loader is no able to get the workspace implementation which will be NULL and this will produce a NULL pointer exception later.

To reproduce this error just add some WPS specific configuration for any workspace.

Related issues:
https://osgeo-org.atlassian.net/browse/GEOS-7253
https://osgeo-org.atlassian.net/browse/GEOS-7159