Bugfix: Prevent NPE if no style store is referenced in auto layer creation

Currently the Workspace.getResource defines that "id" is not null, so creating a autolayer for coverage (or feature) creates a NPE if no stylestore is referenced.

This pull prevents the NPE so the layer will be created like stated in the webservices handbook. 

References:
[Workspace.getResource(...)](/deegree/deegree3/blob/master/deegree-core/deegree-core-workspace/src/main/java/org/deegree/workspace/Workspace.java#L112)
[Hanbook Autolayer of Feature](/deegree/deegree3/blob/master/deegree-services/deegree-webservices-handbook/src/main/sphinx/layers.rst#auto-layers)
[Hanbook Autolayer of Coverage](/deegree/deegree3/blob/master/deegree-services/deegree-webservices-handbook/src/main/sphinx/layers.rst#auto-layers-1)