Fix missing SegmentDescription in PydicomReader (#6937)

Fixes #6928.

### Description

`SegmentDescription` is optional,
https://dicom.innolitics.com/ciods/segmentation/segmentation-image/00620002/00620006,
so try
[`SegmentLabel`](https://dicom.innolitics.com/ciods/segmentation/segmentation-image/00620002/00620005)
first.



### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: KumoLiu <[email protected]>

monai/data/image_reader.py

@@ -766,7 +766,8 @@ def _get_seg_data(self, img):
             all_segs = np.zeros([*spatial_shape, n_classes])
 
         for i, (frames, description) in enumerate(self._get_frame_data(img)):
-            class_name = description.SegmentDescription
+            segment_label = getattr(description, "SegmentLabel", f"label_{i}")
+            class_name = getattr(description, "SegmentDescription", segment_label)
             if class_name not in metadata["labels"].keys():
                 metadata["labels"][class_name] = i
             class_num = metadata["labels"][class_name]