fix projection of point cloud with no positions attribute to image

[rxba]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes project_to_depth_image ERROR Open3D version 0.18 #6651
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Calling project_to_depth_image or project_to_rgbd_image on a o3d.t.geometry.PointCloud() with no positions attribute only throws a non-descriptive IndexError: unordered_map::at: key not found.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

I've prevented the failing call to kernel::pointcloud::Project() with an empty GetPointPositions() by returning an empty image if the point cloud project_to_depth_image or project_to_rgbd_image is called on does not contain the positions attribute.
Additionally, I've added a warning to inform the user of this, and to indicate to them that something else might be going wrong if they are calling these functions without positions.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[rxba]

Not sure why some of the checks are failing, would be glad for some pointers on how to proceed by the experienced contributors. Thanks!

[ssheorey]

Thanks @rxba . Looks good!