Review and Adjust Sorting Logic and Cursor Management in contributions.ts

[coderabbitai]

There is a potential issue with the sorting logic and cursor management in the contributions.ts file, specifically related to handling items with a null 'plantDate' and cursor management for gifts. This issue was identified during a pull request review.

The sorting logic is intended to place items with a null 'plantDate' at the beginning, but there seems to be a contradiction in the implementation. Additionally, the cursor management for gifts does not handle null 'plantDate' values effectively, potentially setting the cursor to null instead of a meaningful value.

For more context, see the pull request and the specific comment:

• PR: Adds group treecounter support for Myforest #2040
• Comment: Adds group treecounter support for Myforest #2040 (comment)

This issue needs further investigation and potential adjustments to align with the intended functionality.