[#51241141] Pass undef from govuk::app
[#51241141] Pass undef from govuk::app

Pass `undef` (literally no value) for `unicorn_herder_timeout` between
`govuk::app` and `govuk::app::config`. This will fallback to the `NOTSET`
value within `::config` which has special meaning therein. It doesn't have
any meaning to the passing class, so omit it.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet