[Bug]: Problem exporting an unsual index (IFNULL expression)

The failure is reported through vendor/phpmyadmin/sql-parser/src/Context.php:608 and Components/Key.php:203 during the ExportSql.php export path. Reproduce it with the supplied drafts table and index definition, then inspect the parser/build path; done means export no longer throws and preserves IFNULL(touser,'CHAT') rather than emitting CHAT.

Describe the bug

I added this index to a table:

ALTER TABLE drafts
  ADD UNIQUE unique_draft (type,user,(IFNULL(touser,'_CHAT_'))) USING BTREE;

and when I try to export it fails with this error (on hosting phpmyadmin version 5.1.0, mysql is Percona 8.0.43-34):

Fatal error: Uncaught TypeError: str_replace(): Argument phpmyadmin/phpmyadmin#3 ($subject) must be of type array|string, null given in /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Context.php:608
Stack trace:
#0 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Context.php(608): str_replace()
phpmyadmin/phpmyadmin#1 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Components/Key.php(203): PhpMyAdmin\SqlParser\Context::escape()
phpmyadmin/phpmyadmin#2 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Component.php(74): PhpMyAdmin\SqlParser\Components\Key::build()
phpmyadmin/phpmyadmin#3 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Components/CreateDefinition.php(360): PhpMyAdmin\SqlParser\Component->__toString()
phpmyadmin/phpmyadmin#4 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Component.php(74): PhpMyAdmin\SqlParser\Components\CreateDefinition::build()
phpmyadmin/phpmyadmin#5 [internal function]: PhpMyAdmin\SqlParser\Component->__toString()
phpmyadmin/phpmyadmin#6 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Components/CreateDefinition.php(339): implode()
phpmyadmin/phpmyadmin#7 /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Statements/CreateStatement.php(380): PhpMyAdmin\SqlParser\Components\CreateDefinition::build()
phpmyadmin/phpmyadmin#8 /var/www/html/phpMyAdmin/libraries/classes/Plugins/Export/ExportSql.php(2951): PhpMyAdmin\SqlParser\Statements\CreateStatement->build()
phpmyadmin/phpmyadmin#9 /var/www/html/phpMyAdmin/libraries/classes/Plugins/Export/ExportSql.php(1638): PhpMyAdmin\Plugins\Export\ExportSql->replaceWithAliases()
phpmyadmin/phpmyadmin#10 /var/www/html/phpMyAdmin/libraries/classes/Plugins/Export/ExportSql.php(2138): PhpMyAdmin\Plugins\Export\ExportSql->getTableDef()
phpmyadmin/phpmyadmin#11 /var/www/html/phpMyAdmin/libraries/classes/Export.php(813): PhpMyAdmin\Plugins\Export\ExportSql->exportStructure()
phpmyadmin/phpmyadmin#12 /var/www/html/phpMyAdmin/libraries/classes/Controllers/ExportController.php(549): PhpMyAdmin\Export->exportDatabase()
phpmyadmin/phpmyadmin#13 /var/www/html/phpMyAdmin/libraries/classes/Routing.php(186): PhpMyAdmin\Controllers\ExportController->index()
phpmyadmin/phpmyadmin#14 /var/www/html/phpMyAdmin/index.php(50): PhpMyAdmin\Routing::callControllerForRoute()
phpmyadmin/phpmyadmin#15 {main}
thrown in /var/www/html/phpMyAdmin/vendor/phpmyadmin/sql-parser/src/Context.php on line 608

On version 5.2.1 with mysql 8.0.41 it won't fails but the export result is like this

ALTER TABLE drafts ADD UNIQUE KEY unique_draft (type,user,(_CHAT_));

which is wrong.

How to Reproduce

Table structure is:

CREATE TABLE drafts (
  type enum('CHAT','MSGS') CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL,
  user varchar(100) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL,
  touser varchar(50) COLLATE utf8mb4_general_ci DEFAULT NULL,
  text text CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL,
  date datetime NOT NULL
);
ALTER TABLE drafts ADD UNIQUE unique_draft (type, user,(IFNULL(touser,'_CHAT_'))) USING BTREE;

Expected behavior

I expect it to export the index like the original one

Screenshots

No response

Operating System

No response

Web Server

No response

Database Server

MySQL

Database version

No response

PHP version

No response

phpMyAdmin version

5.1.0

browser

No response

Additional context

No response