修复局域网连接返回为null问题

backend/internal/service/device_service.go

@@ -111,7 +111,7 @@ func (s *DeviceService) ListCandidates(currentDeviceID string) []model.Device {
 	current, _ := s.store.GetDevice(currentDeviceID)
 	currentNetworkGroupKey := strings.TrimSpace(current.NetworkGroupKey)
 	if currentNetworkGroupKey == "" {
-		return nil
+		return []model.Device{}
 	}
 
 	devices := s.store.ListDevices()