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

@@ -103,10 +103,14 @@ public class MailUtils {
     public static Map<String,Object> sendMails(Collection<String> receivers, Collection<String> receiversCc, String title, String content, ShowType showType) {
         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)));
-
+        
         if (showType == ShowType.TABLE || showType == ShowType.TEXT){
             // send email
             HtmlEmail email = new HtmlEmail();