[PR #921/e234a2e0 backport][0.24] Fix username=None handling

[patchback[bot]]

This is a backport of PR #921 as merged into main (e234a2e020215b3d7ad02230341f7fa078933c3c).

Passing username=None to api_kwargs now behaves as if it was not provided at all.

[noissue]

Review Checklist:

• [ ] An issue is properly linked. [feature and bugfix only]
• [ ] Tests are present or not feasible.
• [ ] Commits are split in a logical way (not historically).