function required(value) {
  return value ? undefined : 'Required'
}
Returns error message if value does not exist, otherwise returns
undefined
@param {string} value - Email to validate
@returns {string|undefined} Required string if value is undefined
@example <caption>Required Field</caption>
<Field
  name="password"
  component={TextField}
  label="Password"
  type="password"
  validate={required}
/>