Avoid query error by adding initial instance to AlarmTrend (#1713)

oap-server/server-query-plugin/query-graphql-plugin/src/main/java/org/apache/skywalking/oap/query/graphql/type/AlarmTrend.java

@@ -18,8 +18,9 @@
 
 package org.apache.skywalking.oap.query.graphql.type;
 
+import java.util.ArrayList;
 import java.util.List;
 
 public class AlarmTrend {
-    private List<Integer> numOfAlarm;
+    private List<Integer> numOfAlarm = new ArrayList<>();
 }