PR: Remove ability to pass illuminant in "CIE XYZ" to "CIE xyY" related transformations.

[KelSolaar]

Summary

This one has been itching me for a while, it is a very old behaviour that is very strongly opinionated but not recommended by CIE during the computation of chromaticity coordinates: When the tristimulus values are null, we do set the chromaticity coordinates to the passed illuminant ones and the luminance to zero. This is what Bruce Lindbloom is doing. It is quite useful as it means that the achromatic colours are all on the same axis but I reckon that this is a behaviour that should be controlled by the user post-conversion. We do not do that with any other space for that matter.

Preflight

Code Style and Quality

• Unit tests have been implemented and passed.
• Pyright static checking has been run and passed.
• Pre-commit hooks have been run and passed.
• [N/A] New transformations have been added to the Automatic Colour Conversion Graph.
• [N/A] New transformations have been exported to the relevant namespaces, e.g. colour, colour.models.

Documentation

• [N/A] New features are documented along with examples if relevant.
• The documentation is Sphinx and numpydoc compliant.

[coveralls]

Coverage: 99.786% (-5.0e-05%) from 99.786% when pulling ef449fd on feature/cie_xyy into 93deeab on develop.

[KelSolaar]

Merging this one, as it improves consistency across the board, especially with regard to the few other space that do projective transformations.