Merge pull request #35 from Baoqi/dev_dont_send_email_when_no_recipts

before send mail, check whether no receivers && cc receivers

escheduler-alert/src/main/java/cn/escheduler/alert/utils/MailUtils.java

@@ -104,6 +104,10 @@ public class MailUtils {
         Map<String,Object> retMap = new HashMap<>();
         retMap.put(Constants.STATUS, false);
         
+        // if there is no receivers && no receiversCc, no need to process
+        if (CollectionUtils.isEmpty(receivers) && CollectionUtils.isEmpty(receiversCc)) {
+            return retMap;
+        }
 
         receivers.removeIf((from) -> (StringUtils.isEmpty(from)));