Changes to List all tax documents

added the new F1099_B enum value to the items/document_subtype response property for the response status 200

added the new F1099_K enum value to the items/document_subtype response property for the response status 200

the response property items/file became nullable for the status 200

the response property items/md5sum became nullable for the status 200

the response property items/year became nullable for the status 200