Improve demo for Sentinel cluster flow control

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-demo/sentinel-demo-cluster/sentinel-demo-cluster-embedded/src/main/java/com/alibaba/csp/sentinel/demo/cluster/init/DemoClusterInitFunc.java

@@ -16,6 +16,7 @@
 package com.alibaba.csp.sentinel.demo.cluster.init;
 
 import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 
 import com.alibaba.csp.sentinel.cluster.ClusterStateManager;
@@ -157,6 +158,7 @@ public class DemoClusterInitFunc implements InitFunc {
         // Otherwise it's unassigned, should be set to NOT_STARTED.
         boolean canBeClient = groupList.stream()
             .flatMap(e -> e.getClientSet().stream())
+            .filter(Objects::nonNull)
             .anyMatch(e -> e.equals(getCurrentMachineId()));
         return canBeClient ? ClusterStateManager.CLUSTER_CLIENT : ClusterStateManager.CLUSTER_NOT_STARTED;
     }

sentinel-demo/sentinel-demo-cluster/sentinel-demo-cluster-server-alone/pom.xml

@@ -35,5 +35,10 @@
             <groupId>com.alibaba.csp</groupId>
             <artifactId>sentinel-datasource-nacos</artifactId>
         </dependency>
+        <dependency>
+            <groupId>ch.qos.logback</groupId>
+            <artifactId>logback-classic</artifactId>
+            <version>1.2.3</version>
+        </dependency>
     </dependencies>
 </project>