Fix task collection/result

[mbardelmeijer]

Changes

• Fix UUID casing for task collections results endpoint.
• Make task result message nullable.

Checks

• [x] The version constant is updated in Client.php
• [x] The changelog is updated (when applicable)
• [x] The supported Cluster API version is updated in the README (when applicable)

[dvdheiden]

Thank you for your contribution @mbardelmeijer !