[2.2.x] Fix nacosServiceInstance scheme npe (#3545)

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

@@ -68,6 +68,11 @@ public class NacosServiceInstance implements ServiceInstance {
 		return metadata;
 	}
 
+	@Override
+	public String getScheme() {
+		return getUri().getScheme();
+	}
+
 	public void setServiceId(String serviceId) {
 		this.serviceId = serviceId;
 	}