code: public function run(): void
      {
          $this->connection->query('
              ALTER TABLE
                  tl_member
              CHANGE
                  username username varchar(64) COLLATE utf8_bin NULL
          ');
  
          $this->connection->query("
              UPDATE
                  tl_member
              SET
                  username = NULL
              WHERE
                  username = ''
          ");
  
          $this->connection->query('
              ALTER TABLE
                  tl_member
              DROP INDEX
                  username,
              ADD UNIQUE KEY
                  username (username)
          ');
      }
docs: {@inheritdoc}