fix PARTSEG-DM

Author: Czaki

package/PartSegCore/segmentation/segmentation_algorithm.py

@@ -376,7 +376,7 @@ def calculation_run(self, report_fun: Callable[[str, int], None]) -> ROIExtracti
 
         report_fun("Flow calculation", 5)
         sprawl_algorithm: BaseWatershed = flow_dict[self.new_parameters["flow_type"]["name"]]
-        mean_brightness = np.mean(cell_channel[cell_mask])
+        mean_brightness = np.mean(cell_channel[cell_mask > 0])
         if mean_brightness < cell_thr:
             mean_brightness = cell_thr + 10
         segmentation = sprawl_algorithm.sprawl(