o3d.geometry.Octree.to_voxel_grid() produces empty VoxelGrid

**Describe the bug**
When trying to convert a valid Octree (initially created from a pointcloud), the resulting Voxelgrid is empty (i.e. contains only one voxel). The pointcloud and the Octree are both valid (verified by rendering them in Open3D).

Is this a bug or am I using it wrong?

**To Reproduce**
Steps to reproduce the behavior:

```
pointcloud = o3d.io.read_point_cloud('path/to/ply')
octree = o3d.geometry.Octree(max_depth=20)
voxelgrid = octree.convert_from_point_cloud(pointcloud)
Out[1]: VoxelGrid with 1 voxels.
```

**Expected behavior**
A voxelgrid matching the initial pointcloud.

**Environment (please complete the following information):**

- Operating system: Windows 10 64-bit
- Python version: 3.7
- Open3D version: 0.12.0 (via pip) and current master (compiled from source)