fix: resolve local source run frontend errors

backend/crm/src/main/java/cn/cordys/crm/system/notice/sse/SseService.java

@@ -49,7 +49,7 @@ public class SseService {
         // 过滤掉默认 admin 用户
         if (StringUtils.isAnyBlank(userId, clientId) || "admin".equals(userId)) {
             log.info("User ID or Client ID is blank, cannot add client.");
-            return null;
+            return Flux.just("HEARTBEAT: " + System.currentTimeMillis()).concatWith(Flux.never());
         }
         Map<String, ClientSinkWrapper> inner = userClients.computeIfAbsent(userId,
                 k -> Collections.synchronizedMap(new LinkedHashMap<>()));