Add redis repository config support

- Change machine id as empty string instead of null
  if it's not set.
- Implement domain classes for redis.
- Shared tests.
- Fixes #267

spring-statemachine-data/jpa/src/main/java/org/springframework/statemachine/data/jpa/JpaRepositoryState.java

@@ -72,6 +72,7 @@ public class JpaRepositoryState extends RepositoryState {
 	 * Instantiates a new jpa repository state.
 	 */
 	public JpaRepositoryState() {
+		this(null);
 	}
 
 	/**
@@ -80,7 +81,7 @@ public class JpaRepositoryState extends RepositoryState {
 	 * @param state the state
 	 */
 	public JpaRepositoryState(String state) {
-		this.state = state;
+		this(state, false);
 	}
 
 	/**
@@ -129,7 +130,7 @@ public class JpaRepositoryState extends RepositoryState {
 	 */
 	public JpaRepositoryState(String machineId, JpaRepositoryState parentState, String state, boolean initial, Set<JpaRepositoryAction> stateActions,
 			Set<JpaRepositoryAction> entryActions, Set<JpaRepositoryAction> exitActions) {
-		this.machineId = machineId;
+		this.machineId = machineId == null ? "" : machineId;
 		this.parentState = parentState;
 		this.state = state;
 		this.initial = initial;

spring-statemachine-data/jpa/src/main/java/org/springframework/statemachine/data/jpa/JpaRepositoryTransition.java

@@ -102,7 +102,7 @@ public class JpaRepositoryTransition extends RepositoryTransition {
 	 * @param actions the actions
 	 */
 	public JpaRepositoryTransition(String machineId, JpaRepositoryState source, JpaRepositoryState target, String event, Set<JpaRepositoryAction> actions) {
-		this.machineId = machineId;
+		this.machineId = machineId == null ? "" : machineId;
 		this.source = source;
 		this.target = target;
 		this.event = event;