Adding gpu box tensors and no points in add_new_points_or_box does not work

When passing a tensor that resides on the gpu to the box argument of add_new_points_or_box and not adding any points, the function raises an error:

> File "/home2/steven/Software/segment-anything-2/sam2/sam2_video_predictor.py", line 226, in add_new_points_or_box
points = torch.cat([box_coords, points], dim=1)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu! (when checking argument for argument tensors in method wrapper_CUDA_cat)

This happens because the points tensor is initialized on the cpu when the points argument is none:
```
if points is None:
points = torch.zeros(0, 2, dtype=torch.float32)
```