Fix returning possible null values

The function may return null values but the return type annotation restricts to return strings only. This workaround prevents PHP from crashing by returning an empty string instead - which is fine because the result is sent to output.

Command/ListClientsCommand.php

@@ -115,7 +115,7 @@ private function getRows(array $clients, array $columns): array
             ];
 
             return array_map(static function (string $column) use ($values): string {
-                return $values[$column];
+                return $values[$column] ?? '';
             }, $columns);
         }, $clients);
     }