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

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

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Operating System

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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

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此故障會在 ExportSql.php 的匯出路徑中，透過 vendor/phpmyadmin/sql-parser/src/Context.php:608 和 Components/Key.php:203 回報。使用提供的 drafts 資料表和索引定義重現此故障，然後檢查 parser/build 路徑；完成標準是匯出不再擲出例外，並保留 IFNULL(touser,'CHAT')，而不是輸出 CHAT。