server: Fix NPE when no recipients configured for sending alerts (#5154)

* Fix NPE when no recipients configured for sending alerts

* Adding logs

* Address comments

Author: Pearl1594

server/src/main/java/com/cloud/alert/AlertManagerImpl.java

@@ -712,6 +712,11 @@ public void sendAlert(AlertType alertType, long dataCenterId, Long podId, Long c
             return;
         }
 
+        if (recipients == null) {
+            s_logger.warn(String.format("No recipients set in 'alert.email.addresses', skipping sending alert with subject: %s and content: %s", subject, content));
+            return;
+        }
+
         SMTPMailProperties mailProps = new SMTPMailProperties();
         mailProps.setSender(new MailAddress(senderAddress));
         mailProps.setSubject(subject);