How to use the yup.nullableString function in yup

// limitations under the License.

import * as Yup from 'yup'

import randomByteString from '../../../lib/random-bytes'
import m from '../../../components/device-data-form/messages'
import sharedMessages from '../../../../lib/shared-messages'

import { parseLorawanMacVersion } from '../utils'

const random16BytesString = () => randomByteString(32)
const toUndefined = value => (!Boolean(value) ? undefined : value)

const validationSchema = Yup.object()
  .shape({
    net_id: Yup.nullableString()
      .emptyOrLength(3 * 2, m.validate6) // 3 Byte hex
      .default(''),
    root_keys: Yup.object().when(
      ['_external_js', '_lorawan_version'],
      (externalJs, version, schema) => {
        const strippedSchema = Yup.object().strip()
        const keySchema = Yup.lazy(() => {
          return !externalJs
            ? Yup.object().shape({
                key: Yup.string()
                  .emptyOrLength(16 * 2, m.validate32) // 16 Byte hex
                  .transform(toUndefined)
                  .default(random16BytesString),
              })
            : Yup.object().strip()
        })

gatewayNamePlaceholder: 'My New Gateway',
  gsServerAddressDescription: 'The address of the Gateway Server to connect to',
  gatewayDescPlaceholder: 'Description for my new gateway',
  gatewayDescDescription:
    'Optional gateway description; can also be used to save notes about the gateway',
})

const validationSchema = Yup.object().shape({
  owner_id: Yup.string(),
  ids: Yup.object().shape({
    gateway_id: Yup.string()
      .matches(gatewayIdRegexp, sharedMessages.validateAlphanum)
      .min(2, sharedMessages.validateTooShort)
      .max(36, sharedMessages.validateTooLong)
      .required(sharedMessages.validateRequired),
    eui: Yup.nullableString().length(8 * 2, sharedMessages.validateTooShort),
  }),
  name: Yup.string()
    .min(2, sharedMessages.validateTooShort)
    .max(50, sharedMessages.validateTooLong),
  description: Yup.string().max(2000, sharedMessages.validateTooLong),
  frequency_plan_id: Yup.string().required(sharedMessages.validateRequired),
  gateway_server_address: Yup.string().matches(addressRegexp, sharedMessages.validateAddressFormat),
})

@bind
class GatewayDataForm extends React.Component {
  onSubmit(values, helpers) {
    const { onSubmit } = this.props
    const castedValues = validationSchema.cast(values)

    onSubmit(castedValues, helpers)