Fix applicationForm.submit Rejecting Null utmData Fields

applicationForm.submit now correctly accepts null for utmData and its individual fields (utm_source, utm_medium, utm_campaign, utm_term, utm_content) again. Sending {"utmData": {"utm_source": null}} or {"utmData": null} was returning invalid_input after Zod validation was added in the presigned file upload release on April 14; this has been fixed. Omitting utmData or passing valid string values continues to work as before.