fix issue#14441 (#14441) (#14444)

alert-plugin-instances api, if no alert-plugin-instances, it will response an empty list, but not null

Co-authored-by: ligen60 <ligen60@jd.com>

dolphinscheduler-api/src/main/java/org/apache/dolphinscheduler/api/service/impl/AlertPluginInstanceServiceImpl.java

@@ -243,16 +243,16 @@ public class AlertPluginInstanceServiceImpl extends BaseServiceImpl implements A
     }
 
     private List<AlertPluginInstanceVO> buildPluginInstanceVOList(List<AlertPluginInstance> alertPluginInstances) {
+        List<AlertPluginInstanceVO> alertPluginInstanceVOS = new ArrayList<>();
         if (CollectionUtils.isEmpty(alertPluginInstances)) {
-            return null;
+            return alertPluginInstanceVOS;
         }
         List<PluginDefine> pluginDefineList = pluginDefineMapper.queryAllPluginDefineList();
         if (CollectionUtils.isEmpty(pluginDefineList)) {
-            return null;
+            return alertPluginInstanceVOS;
         }
         Map<Integer, PluginDefine> pluginDefineMap =
                 pluginDefineList.stream().collect(Collectors.toMap(PluginDefine::getId, Function.identity()));
-        List<AlertPluginInstanceVO> alertPluginInstanceVOS = new ArrayList<>();
         alertPluginInstances.forEach(alertPluginInstance -> {
             AlertPluginInstanceVO alertPluginInstanceVO = new AlertPluginInstanceVO();