fix: Record validation errors

[ReubenFrankel]

When running against an SDK target with validate_records set to true, we hit a couple of record validation errors:

• line_items

  jsonschema.exceptions.ValidationError: [] is not of type 'object', 'null'
  
  Failed validating 'type' in schema['properties']['properties']:
      {'additionalProperties': True,
       'properties': {},
       'type': ['object', 'null']}
  
  On instance['properties']:
      []

• orders

  jsonschema.exceptions.ValidationError: False is not of type 'string', 'null'
  
  Failed validating 'type' in schema['properties']['tax_exempt']:
      {'type': ['string', 'null']}
  
  On instance['tax_exempt']:
      False