优化参数获取

jetlinks-components/dashboard-component/src/main/java/org/jetlinks/community/dashboard/MeasurementParameter.java

@@ -18,7 +18,7 @@ public class MeasurementParameter implements ValueObject {
     private Map<String, Object> params = new HashMap<>();
 
     public Optional<Object> get(String name) {
-        return Optional.ofNullable(params.get(name));
+        return Optional.ofNullable(params).map(p -> p.get(name));
     }
 
 }