Make base_path optional for nested links
Make base_path optional for nested links

As described in
https://github.com/alphagov/govuk_publishing_components/pull/595#discuss
ion_r229322211,
https://github.com/alphagov/govuk-content-schemas/pull/818 made the
`base_path` optional in the `links` for certain schemas. However, those
same schemas will still need a `base_path` if they're in nested links.

alphagov/govuk-content-schemas,alphagov/govuk-content-schemas