Fix parent state ref

- For RepositoryState, its parent state is now
  defined as RepositoryState as well allowing
  to reference from a json files.
- Relates to #262

spring-statemachine-data/src/main/java/org/springframework/statemachine/data/RepositoryState.java

@@ -30,7 +30,7 @@ public abstract class RepositoryState extends BaseRepositoryEntity {
 	 *
 	 * @return the parent state
 	 */
-	public abstract String getParentState();
+	public abstract RepositoryState getParentState();
 
 	/**
 	 * Gets the machine id.

spring-statemachine-data/src/main/java/org/springframework/statemachine/data/RepositoryStateMachineModelFactory.java

@@ -132,7 +132,9 @@ public class RepositoryStateMachineModelFactory extends AbstractStateMachineMode
 				}
 			}
 
-			StateData<String,String> stateData = new StateData<String, String>(s.getParentState(), null, s.getState(), s.isInitial());
+			RepositoryState parentState = s.getParentState();
+			StateData<String, String> stateData = new StateData<String, String>(parentState != null ? parentState.getState() : null, null,
+					s.getState(), s.isInitial());
 			stateData.setStateActions(stateActions);
 			stateData.setEntryActions(entryActions);
 			stateData.setExitActions(exitActions);