changed output of colored icp to be identity if no correpondence is found

[bkhanal-11]

When correspondence between source and target point cloud is not found, generalized icp returns their transformation to be identity. Updated this idea to colored icp.

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This change is 

[update-docs[bot]]

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

[scimad]

@bkhanal-11 , I think the verbosity level WARNING might be more appropriate.

[reyanshsolis]

I would highly recommend using the new API. The legacy colored-ICP API has some issues, but it is going to be depreciated soon anyways. Documentation: http://www.open3d.org/docs/latest/tutorial/t_pipelines/t_icp_registration.html#Case-of-no-correspondences.

Thanks

[bkhanal-11]

Thank you for letting me know. I will close this then.