If do not specify strict_mode explicitly, do not set sql_mode.

Related https://github.com/rails/rails/issues/17370
Because the next line is `variables['sql_mode'] = strict_mode? ? 'STRICT_ALL_TABLES' : ''`,
when `strict: true` it becomes `variables['sql_mode'] = 'STRICT_ALL_TABLES'`,
when `strict: false` it becomes `variables['sql_mode'] = ''`.