New blogs miss the default categories whose legacy page is gone from the Blog space

When a blog is created from Blog.Management with "Create default categories" checked, its default categories (News, Other, Personal) are copies of the legacy category pages of the Blog space (Blog.News, Blog.Other, Blog.Personal) instead of the categories of the default blog, which live in Blog.Categories since 9.15.

Once a legacy page is gone (the categories migration moves them onto Blog.Categories.*, see BLOG-275, and BLOG-280 plans to stop shipping them), the corresponding default category is not created.

Steps to reproduce

1. Delete Blog.Other (or migrate the blog categories).
2. Create a new blog from Blog.Management with "Create default categories" checked.
3. Manage the categories of the new blog.

Expected

The new blog has the categories News, Other and Personal.

Actual

It has only News and Personal. An empty page named Other (without category object) is created in its categories space instead.

Cause

Management.xml:114 copies "Blog.${categoryName}"; when the copy fails, lines 115-117 still save an empty page with the category name.

Pinned by the functional test BlogManagementIT#createBlogWithADotInItsNameAndItsCategoriesAtTheTopLevel (BLOG-269).