fix: NotificationListener 수신자 Null-safe 방어 로직 추가

backend/src/main/java/com/srsystem/listener/NotificationListener.java

@@ -29,8 +29,16 @@ public class NotificationListener {
     @Async
     @TransactionalEventListener
     public void handleNotification(NotificationEvent event) {
-        Employee recipient = employeeRepository.findById(event.getRecipientId())
-                .orElseThrow(() -> new BusinessException(ErrorCode.EMPLOYEE_NOT_FOUND));
+        if (event == null || event.getRecipientId() == null) {
+            log.warn("NotificationEvent is null or recipientId is missing: {}", event);
+            return;
+        }
+
+        Employee recipient = employeeRepository.findById(event.getRecipientId()).orElse(null);
+        if (recipient == null) {
+            log.warn("Recipient employee not found for id: {}. Notification skipped.", event.getRecipientId());
+            return;
+        }
 
         Notification notification = Notification.builder()
                 .user(recipient)