Multisite, Settings: Prevent saving an empty site title or network title via admin screens

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/65718

[github-actions]

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[Copilot]

Pull request overview

This PR prevents administrators from saving an empty Site Title (blogname) or Network Title (site_name) through the relevant admin screens, aligning edit behavior with creation-time requirements (notably in Multisite).

Changes:

• Add server-side validation in sanitize_option() to reject empty/whitespace-only blogname updates (retaining the stored value and registering a settings error).
• Mark Site Title / Network Title inputs as required in the General Settings and Network Settings forms.
• In Multisite site settings, carry sanitize_option() settings errors across the redirect to display an error notice instead of a success message.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/wp-includes/formatting.php	Rejects empty Site Title updates during option sanitization and registers a settings error.
src/wp-admin/options-general.php	Marks “Site Title” as a required field in the General Settings UI.
src/wp-admin/network/site-settings.php	Persists settings errors across redirect and displays error notices when site options aren’t saved.
src/wp-admin/network/settings.php	Prevents saving an empty Network Title, shows an error notice, and marks the field as required.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

src/wp-includes/formatting.php:5003

• This change introduces new behavior for sanitize_option( 'blogname', ... ) (rejecting empty/whitespace-only values and registering a settings error), but there’s no PHPUnit coverage asserting the new validation and the expected get_settings_errors( 'blogname' ) result. Adding a unit test will help prevent regressions in both the returned value (should remain the previously stored option) and the presence/content of the settings error.

				// The site title (blogname) cannot be empty.
				if ( 'blogname' === $option && '' === trim( $value ) ) {
					$error = __( 'The site title cannot be empty. Please enter a title for your site.' );
				}