test(bpmn-model): add test for null attribute list

- add test for null and empty attribute string lists
- move string util tests to xml-model

related to #CAM-2256
fix(OutputSet): correct return types of OutputSet

related to #CAM-2293
fix(bpmn-model): add IntermediateThrowEvent

related to #CAM-2298
fix(bpmn-model): add missing event definitions except escalation

- add CancelEventDefinition
- add CompensateEventDefinition
- add ConditionalEventDefinition
- add ErrorEventDefinition
- add LinkEventDefinition
- add SignalErrorEventDefinition
- add TerminateEventDefinition
- add TimerEventDefinition

related to #CAM-2275
feat(bpmn-model): add escalation and escalation event definition

related to #CAM-2339
fix(bpmn-model): add missing gateway types

- add ComplexGateway
- add EventBasedGateway
- add InclusiveGateway

related to #CAM-2286
test(bpmn-di): add failing test case for remove of reference

related to #CAM-2276
fix(bpmndi): add BPMNDiagram to definitions and remove namespaces

- add missing BPMNDiagram collection to definitions element
- remove all attribute namespaces for BPMNDI, DC and DI to create
  valid BPMNDI elements

related to #CAM-2277