Code Modernization: Replace void in PHPDoc union return types with null in Administration

[apermo]

Following up on #64694 which fixed paginate_links(), this ticket addresses 13 instances across Administration files (class-wp-admin-bar.php and wp-admin/includes/) where @return annotations incorrectly use void as part of a union type.

The |void pattern in these files was introduced across multiple versions from WordPress 4.3 through 6.8, with the most recent being column_status() in class-wp-privacy-requests-table.php added in WP 6.8 via [59336].

Why this matters

In PHP's type system, void means "the function does not return a value" and cannot be part of a union type (this is a compile error in PHP 8.0+). When a function uses bare return; or falls off the end without returning, the actual runtime value is null. Therefore @return Type|void should be @return Type|null.

This affects:

• Static analysis tools (PHPStan, Psalm)
• IDE autocompletion and type inference
• Developer expectations about return values

Backward Compatibility

This is a safe, non-breaking change. As demonstrated by @westonruter in the ​#64694 PR review, return; and return null; are identical at runtime across every PHP version from 4.3.0 through 8.5.3: ​https://3v4l.org/3KQC8

Proposed Changes

For each instance:

1. Update the @return annotation to replace void with null (or remove |void where the function always returns a typed value)
2. Change bare return; statements to return null;
3. Update description text to say "null" instead of "void" / "nothing"

Affected Functions — class-wp-admin-bar.php (5)

Function	Line	Current	Recommendation	Since
get_node()	196	object|void	object|null	5.3
_get_node()	209	object|void	object|null	4.3
get_nodes()	228	array|void	array|null	4.3
_get_nodes()	245	array|void	array|null	4.3
_bind()	310	object|void	object|null	4.3

Affected Functions — wp-admin/includes/ (8)

File	Function	Line	Current	Recommendation	Since
class-wp-importer.php	set_blog()	139	int|void	int (void incorrect)	4.3
class-wp-importer.php	set_user()	180	int|void	int (void incorrect)	4.3
class-wp-site-health.php	perform_test()	170	mixed|void	mixed (void incorrect)	5.4
post.php	write_post()	985	int|void	int (void incorrect)	5.7
media.php	media_upload_form_handler()	739	null|array|void	null|array (void redundant)	5.9
plugin.php	uninstall_plugin()	1299	true|void	true|null	5.7
file.php	validate_file_to_edit()	737	string|void	string (void incorrect)	5.0
dashboard.php	wp_dashboard_quota()	1651	true|void	true|null	5.7
class-wp-privacy-requests-table.php	column_status()	438	string|void	string|null	6.8

See #64694 for prior art.

Co-Authored-By: xateman

Trac ticket: https://core.trac.wordpress.org/ticket/64702

## Use of AI Tools

Used AI for research, documentation and for the replacements. Everything was reviewed by myself and @xateman before opening this PR.