Only prepend hostname if it has text

spring-cloud-netflix-eureka-server/src/main/java/org/springframework/cloud/netflix/eureka/server/CloudJacksonJson.java

@@ -7,6 +7,7 @@ import java.lang.reflect.Field;
 import java.util.HashMap;
 
 import org.springframework.util.ReflectionUtils;
+import org.springframework.util.StringUtils;
 
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.core.JsonGenerator;
@@ -133,7 +134,7 @@ public class CloudJacksonJson extends LegacyJacksonJson {
 		private String calculateInstanceId(InstanceInfo info) {
 			String instanceId = info.getMetadata().get("instanceId");
 			String hostName = info.getHostName();
-			if (instanceId != null && !instanceId.startsWith(hostName)) {
+			if (instanceId != null && StringUtils.hasText(hostName) && !instanceId.startsWith(hostName)) {
 				instanceId = hostName + ":" + instanceId;
 			}
 			return instanceId == null ? hostName : instanceId;