fix NPE when flush logical cluster and physical cluster not in cache or not exist

kafka-manager-core/src/main/java/com/xiaojukeji/kafka/manager/service/cache/LogicalClusterMetadataManager.java

@@ -208,7 +208,8 @@ public class LogicalClusterMetadataManager {
         // 计算逻辑集群到Topic名称的映射
         Set<String> topicNameSet = PhysicalClusterMetadataManager.getBrokerTopicNum(
                 logicalClusterDO.getClusterId(),
-                brokerIdSet);
+                brokerIdSet
+        );
         LOGICAL_CLUSTER_ID_TOPIC_NAME_MAP.put(logicalClusterDO.getId(), topicNameSet);
 
         // 计算Topic名称到逻辑集群的映射

kafka-manager-core/src/main/java/com/xiaojukeji/kafka/manager/service/cache/PhysicalClusterMetadataManager.java

@@ -539,9 +539,12 @@ public class PhysicalClusterMetadataManager {
     }
 
     public static Set<String> getBrokerTopicNum(Long clusterId, Set<Integer> brokerIdSet) {
-        Set<String> topicNameSet = new HashSet<>();
-
         Map<String, TopicMetadata> metadataMap = TOPIC_METADATA_MAP.get(clusterId);
+        if (metadataMap == null) {
+            return new HashSet<>();
+        }
+
+        Set<String> topicNameSet = new HashSet<>();
         for (String topicName: metadataMap.keySet()) {
             try {
                 TopicMetadata tm = metadataMap.get(topicName);