message: Don't serialize relationships if they aren't present
diff: @@ -11,12 +11,17 @@ module.exports = function rowToDocument(schemaModels, type, row) {
  };
  for (let field of fields) {
  if(field.attributes['field-type'] === '@cardstack/core-types::belongs-to') {
  - doc.relationships[field.id] = {
  + let relatedId = row[field.id];
  +
  + if (relatedId) {
  + doc.relationships[snakeCase(field.id)] = {
  data: {
  - id: row[field.id],
  + id: relatedId,
  type: field.relationships['related-types'].data[0].id // TODO: This doesn't support polymorphic relationships
  }
  };
  + }
  +
  } else {
  doc.attributes[field.id] = convertValue(row[snakeCase(field.id)], field.attributes['field-type']);
  }
  