Fix error setting the previous_title link when the overview does not have a section title. See #198.

[finnlewis]

This should fix the fatal error we are seeing on localgovdrupal/localgov_subsites_extras#39

We should follow up as discussed yesterday on harmonising the logic for section titles, whether they are required and how the we. make use of them for previous next navigation links.

CC @markconroy @ekes

Steps to test:

1. Default install of LocalGov Drupal with LocalGov Guides on 2.x branch
2. Create Guide overview page with no section title
3. Create guide page with parent of the overview.
4. Save
5. See error

Drupal\Core\Render\Component\Exception\InvalidComponentException: [prev_title] NULL value found, but a string or an object is required.

6. Checkout this branch
7. No error :)

[ekes]

It's my own hasty test, but fixes the subsites extras one too. So let's go.