Improve doc for WP_Screen class

[Chouby]

WooCommerce 6.9 introduced a conflict with several plugins by accessing to the property WP_Screen::id and expecting it to be a string, while conflicting plugins just set the current screen with a simple call to set_current_screen() without parameter. In this case, get_current_screen() returns a WP_Screen object with the property id set to null.

This pushed me to review the doc of the class and notice that several properties are not correctly documented for this case.

[Chouby]

Just for reference, the issue opened in WooCommerce: ​https://github.com/woocommerce/woocommerce/issues/34725

## Summary

• Correct the documented type of WP_Screen::$_screen_settings.
• Account for the property being null until show_screen_options() initializes it.

The ticket's other proposed type changes have already been addressed by later commits or are no longer applicable to current trunk.

## Testing

• php -l src/wp-admin/includes/class-wp-screen.php
• phpcbf --standard=phpcs.xml.dist src/wp-admin/includes/class-wp-screen.php
• phpcs --standard=phpcs.xml.dist src/wp-admin/includes/class-wp-screen.php

[SergeyBiryukov]

Replying to Chouby:

...accessing to the property WP_Screen::id and expecting it to be a string, while conflicting plugins just set the current screen with a simple call to set_current_screen() without parameter. In this case, get_current_screen() returns a WP_Screen object with the property id set to null.

Thanks for the ticket! Just noting that I cannot reproduce this specific issue in current trunk:

require 'wp-load.php';
require 'wp-admin/includes/admin.php';

set_current_screen();
var_dump( get_current_screen() );

Both WP_Screen::$id and WP_Screen::$base are an empty string in this scenario in my testing.

That said, WP_Screen::$_screen_settings is indeed null and appears to be the only property left to correct here, as the other affected properties have already been adjusted in [55693] and [61300].

[SergeyBiryukov]

In 62837:

Docs: Correct the type for WP_Screen::$_screen_settings.

This reflects the property's initial null state prior to initialization.

Follow-up to [55693], [61300].

Props Chouby, arkaprabhachowdhury, SergeyBiryukov.
Fixes #56607.