🐛 :: 푸쉬알림 대상자 없을 때 에러 발생부분 수정

src/main/java/io/github/depromeet/knockknockbackend/domain/notification/service/NotificationService.java

@@ -172,6 +172,9 @@ public void sendInstance(SendInstanceRequest request) {
 
         List<DeviceToken> deviceTokens = getDeviceTokens(request.getGroupId(), sendUserId);
         List<String> tokens = getFcmTokens(deviceTokens);
+        if (tokens.isEmpty()) {
+            return;
+        }
 
         fcmService.sendGroupMessage(
                 tokens, request.getTitle(), request.getContent(), request.getImageUrl());