Code Quality: Replace `void` with proper return types in union PHPDoc annotations.

In PHP's type system, `void` means a function does not return a value and cannot be part of a union type. Many functions in core were documented as returning e.g. `string|void` while actually returning `null` implicitly via bare `return;` statements. This replaces `void` with `null` in union return types, adds explicit `return null;` statements, and updates `@return` annotations across 22 files in `wp-includes`.

Additionally:
* Adds `@return never` for `WP_Recovery_Mode::redirect_protected()`.
* Fixes `WP_Theme_JSON::set_spacing_sizes()` to use `@return void` instead of `@return null|void`.
* Removes `void` from return types where the function always returns a value or dies: `remove_theme_support()`, `WP_Recovery_Mode::handle_error()`.
* Fixes `wp_die()` return type from `never|void` to `void` with clarified description.
* Initializes `$primary` variable in `get_active_blog_for_user()` to prevent a possible undefined variable notice.

Developed in https://github.com/WordPress/wordpress-develop/pull/11012

Follow-up to r62177, r61766, r61719.

Props apermo, xateman, westonruter, parthvataliya, nimeshatxecurify.
See #64704.


git-svn-id: https://develop.svn.wordpress.org/trunk@62178 602fd350-edb4-49c9-b593-d223f7449a82