Users: Prevent creating of empty usernames after sanitization.

Introduces a check in `wp_insert_user()` to ensure the username doesn't have a length of zero after sanitization removes invalid characters.

Props kalpeshh, missveronicatv, rayhatron, rinkalpagdar, sergeybiryukov, thehercules.
Fixes #57635.



git-svn-id: https://develop.svn.wordpress.org/trunk@60288 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc

src/wp-includes/user.php

@@ -2286,7 +2286,10 @@ function wp_insert_user( $userdata ) {
 	 */
 	$user_nicename = apply_filters( 'pre_user_nicename', $user_nicename );
 
-	if ( mb_strlen( $user_nicename ) > 50 ) {
+	// Check if the sanitized nicename is empty.
+	if ( empty( $user_nicename ) ) {
+		return new WP_Error( 'empty_user_nicename', __( 'Cannot create a user with an empty nicename.' ) );
+	} elseif ( mb_strlen( $user_nicename ) > 50 ) {
 		return new WP_Error( 'user_nicename_too_long', __( 'Nicename may not be longer than 50 characters.' ) );
 	}