kie-dmn-core: PMMLInfo NPE when model has null output

During testing of the WB changes for PMML support jomarko@redhat.com found the example PMML files referenced by our KIE documentation failed to have a number of models listed against their document definition.

On examination PMMLInfo.from(..) throws a NPE if the PMML model lacks an output (see L#57 that throws the NPE if the model has a null output). I've attached the files jomarko@redhat.com used for his testing.