spring-projectsGH-2853: Handle null Cluster ID

Resolves spring-projects#2853

Apparently, some third party Kafka replacements can return a `null` `clusterId`.

This causes an attempt to retrieve it on every operation.

**cherry-pick to 3.0.x**

spring-kafka/src/main/java/org/springframework/kafka/core/KafkaAdmin.java

@@ -321,6 +321,9 @@ public String clusterId() {
 		if (this.clusterId == null) {
 			try (AdminClient client = createAdmin()) {
 				this.clusterId = client.describeCluster().clusterId().get(this.operationTimeout, TimeUnit.SECONDS);
+				if (this.clusterId == null) {
+					this.clusterId = "null";
+				}
 			}
 			catch (InterruptedException ex) {
 				Thread.currentThread().interrupt();
@@ -358,7 +361,7 @@ public Map<String, TopicDescription> describeTopics(String... topicNames) {
 		}
 	}
 
-	private AdminClient createAdmin() {
+	AdminClient createAdmin() {
 		Map<String, Object> configs2 = new HashMap<>(this.configs);
 		checkBootstrap(configs2);
 		return AdminClient.create(configs2);

spring-kafka/src/test/java/org/springframework/kafka/core/KafkaAdminTests.java

@@ -19,6 +19,8 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.awaitility.Awaitility.await;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
 
 import java.lang.reflect.Method;
 import java.util.Arrays;
@@ -37,11 +39,13 @@
 import org.apache.kafka.clients.admin.AlterConfigOp;
 import org.apache.kafka.clients.admin.AlterConfigOp.OpType;
 import org.apache.kafka.clients.admin.ConfigEntry;
+import org.apache.kafka.clients.admin.DescribeClusterResult;
 import org.apache.kafka.clients.admin.DescribeConfigsResult;
 import org.apache.kafka.clients.admin.DescribeTopicsResult;
 import org.apache.kafka.clients.admin.NewPartitions;
 import org.apache.kafka.clients.admin.NewTopic;
 import org.apache.kafka.clients.admin.TopicDescription;
+import org.apache.kafka.common.KafkaFuture;
 import org.apache.kafka.common.config.ConfigResource;
 import org.apache.kafka.common.config.ConfigResource.Type;
 import org.apache.kafka.common.config.TopicConfig;
@@ -268,6 +272,24 @@ void toggleBootstraps() {
 				.isEqualTo("a,b,c");
 	}
 
+	@Test
+	void nullClusterId() {
+		AdminClient mock = mock(AdminClient.class);
+		DescribeClusterResult result = mock(DescribeClusterResult.class);
+		KafkaFuture<String> fut = KafkaFuture.completedFuture(null);
+		given(result.clusterId()).willReturn(fut);
+		given(mock.describeCluster()).willReturn(result);
+		KafkaAdmin admin = new KafkaAdmin(Map.of()) {
+
+			@Override
+			AdminClient createAdmin() {
+				return mock;
+			}
+
+		};
+		assertThat(admin.clusterId()).isEqualTo("null");
+	}
+
 	@Configuration
 	public static class Config {