fix #3966 sub process doesnot send alert mail after process instance ending. (#3972)

Co-authored-by: baoliang <baoliang@analysys.com.cn>

dolphinscheduler-server/src/main/java/org/apache/dolphinscheduler/server/utils/AlertManager.java

@@ -19,6 +19,7 @@ package org.apache.dolphinscheduler.server.utils;
 
 import org.apache.dolphinscheduler.common.enums.AlertType;
 import org.apache.dolphinscheduler.common.enums.CommandType;
+import org.apache.dolphinscheduler.common.enums.Flag;
 import org.apache.dolphinscheduler.common.enums.ShowType;
 import org.apache.dolphinscheduler.common.enums.WarningType;
 import org.apache.dolphinscheduler.common.utils.DateUtils;
@@ -207,6 +208,9 @@ public class AlertManager {
      */
     public void sendAlertProcessInstance(ProcessInstance processInstance,
                                          List<TaskInstance> taskInstances){
+        if(Flag.YES == processInstance.getIsSubProcess()){
+            return;
+        }
 
         boolean sendWarnning = false;
         WarningType warningType = processInstance.getWarningType();