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

Following up on #64694 which fixed paginate_links(), this ticket addresses 8 instances across 4 Customize files where @return annotations incorrectly use void as part of a union type.

The |void pattern in these files was introduced in WordPress 4.3 via [32568] — "Use void instead of null where appropriate when pipe-delimiting @return types". The get_current_image_src() annotation originated in the same commit but was carried to its current file location in WP 4.4 via a file move.

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
2. Change bare return; statements to return null;
3. Update description text to say "null" instead of "void" / "nothing"

Affected Functions

See #64694 for prior art.