Merging empty tensor point cloud fails

[conanomori]

Checklist

• I have searched for similar issues.
• For Python issues, I have tested with the latest development wheel.
• I have checked the release documentation and the latest documentation (for main branch).

Describe the issue

I am trying to merge 2 tensor point clouds but am getting at index error. One point cloud is empty, while another has points. I am trying to do this as I am dynamically merging point clouds from scratch

I have tried both + notation and append function (which I assume performs the same operation, but there is no description in the docs) and both methods fail.

The legacy format accounted for this and allowed you to merge empty point clouds.

Steps to reproduce the bug

import open3d as o3d
import open3d.core as o3c

# legacy point cloud merges successfully with empty point cloud
pcd_1 = o3d.geometry.PointCloud()
pcd_2 = o3d.geometry.PointCloud()
pcd_2.points = o3d.utility.Vector3dVector([[0, 0, 0], [1, 1, 1]])
pcd_1 += pcd_2

# tensor point cloud fails to merge with empty
pcd_3 = o3d.t.geometry.PointCloud()
pcd_4 = o3d.t.geometry.PointCloud(o3c.Tensor([[0, 0, 0], [1, 1, 1]], o3c.float32))
pcd_3 += pcd_4

Error message

IndexError: invalid unordered_map<K, T> key

Expected behavior

Empty tensor point clouds successfully merges with other point clouds without fail

Open3D, Python and System information

- Operating system: Windows 10 64-bit
- Python version: Python 3.11.9
- Open3D version: 0.18.0
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
- Compiler version (if built from source): N/A

Additional information

No response