[Fix] fix local_visualizer.py: only visualize when there is at least one instance (#2231)

* fix local_visualizer.py: modify add_datasample

 - fix "AttributeError: ‘InstanceData’ object has no attribute ‘bboxes_3d’" error

* Update mmdet3d/visualization/local_visualizer.py
Co-authored-by: Xiang Xu <xuxiang0103@gmail.com>

* fix local_visualizer.py: modify add_datasample

 - add exception handling when there is no instance inside pred_instances_3d

* modify local_visualizer.py: Apply changes from pre-commit

* modify local_visualizer.py: Change exception handling location for empty instance

 - Add exception handling when instances are empty
 - Delete the exception handling previously added to add_datasample

* modify local_visualizer.py: apply pre-commit

---------
Co-authored-by: Xiang Xu <xuxiang0103@gmail.com>

mmdet3d/visualization/local_visualizer.py

@@ -557,6 +557,10 @@ class Det3DLocalVisualizer(DetLocalVisualizer):
             dict: the drawn point cloud and image which channel is RGB.
         """
 
+        # Only visualize when there is at least one instance
+        if not len(instances) > 0:
+            return None
+
         bboxes_3d = instances.bboxes_3d  # BaseInstance3DBoxes
 
         data_3d = dict()
@@ -786,6 +790,8 @@ class Det3DLocalVisualizer(DetLocalVisualizer):
                 drawn_img_3d = gt_data_3d['img']
             elif pred_data_3d is not None:
                 drawn_img_3d = pred_data_3d['img']
+            else:  # both instances of gt and pred are empty
+                drawn_img_3d = None
         else:
             drawn_img_3d = None