Objects without `post__date_published` appear first despite descending order

[rprader]

Currently, when accessing the overview of all articles, entries without a post__date_published value appear before those that have one.

Use case:

When creating posts, I am not always specifing a publication date.

• If I publish a post immediately, I leave post__date_published empty — as it’s not required.
• If I plan a post in advance, I explicitly set a post__date_published value.

As a result, immediately published posts (with no date_published) currently appear before the scheduled ones, which is the opposite of what’s expected.

Expected behavior:

The post__date_published field should serve as the primary sorting criterion, and if it’s missing (NULL), the ordering should fall back to post__date_created. In other words, the most recently published posts should appear first, followed by those without a publication date, sorted by their creation time.