DynamicVariablesConfig values restored

What's Changed

• [fix] Restore optional values in DynamicVariablesConfig records (2.53.1) by @PaulAsjes in https://github.com/elevenlabs/elevenlabs-js/pull/411

Full Changelog: https://github.com/elevenlabs/elevenlabs-js/compare/v2.53.0...v2.53.1