GDM-205: extra null check for raster layers without featureinfo

This can cause freeze in uploading WMS layer via wizard.

E.g. for http://grb.agiv.be/geodiensten/raadpleegdiensten/GRB-basiskaart/wms?SERVICE=WMS&version=1.3.0&request=GetCapabilities (any layer)

bug NEEDS VOTE