Blog.ManageCategories doesn't list the categories of the default blog when they are stored in Blog.Categories

Steps to reproduce

• On a fresh install of the Blog application (no blog post uses a legacy category from the Blog space):
• Go to "/xwiki/bin/view/Blog/ManageCategories".
• Click "Add a category", enter a name and click "Add".
• Reload the page.

Expected result

• The page lists the categories of the default blog: News, Other, Personal.
• The new category is listed after the reload.

Actual result

• No category is listed, only the "Add a category" link.
• Right after it is added, the new category appears (the list is refreshed through AJAX), but it disappears after the reload. The category page is created as Blog.Categories.<name>.
• The Categories panel displayed on that page is empty too, and its "All" and RSS links point to "Blog" instead of "Blog.Categories".
• Without JavaScript, the add form redirects back to "Blog.ManageCategories", so the category that was just created is never shown.

"Blog.Categories.WebHome?action=manage" lists the categories correctly. It is also what the "Edit Categories" link of the Categories panel targets when the panel resolves the categories location to "Blog.Categories, as it does on the blog home page.