Merge pull request #1536 from guerlab/master

[issue #1535 ]fix NacosServiceDiscovery.hostToServiceInstance  method potential NullPointerException

spring-cloud-alibaba-starters/spring-cloud-starter-alibaba-nacos-discovery/src/main/java/com/alibaba/cloud/nacos/discovery/NacosServiceDiscovery.java

@@ -92,7 +92,9 @@ public class NacosServiceDiscovery {
 		metadata.put("nacos.weight", instance.getWeight() + "");
 		metadata.put("nacos.healthy", instance.isHealthy() + "");
 		metadata.put("nacos.cluster", instance.getClusterName() + "");
+		if (instance.getMetadata() != null) {
 			metadata.putAll(instance.getMetadata());
+		}
 		nacosServiceInstance.setMetadata(metadata);
 
 		if (metadata.containsKey("secure")) {