Fix error on entity canonical page when there is no site resolver.

[andrewbelcher]

This pull request is for:

• examples/*
• modules/next
• packages/next-drupal
• starters/basic-starter
• starters/graphql-starter
• Other

GitHub Issue: #517

Describe your changes

As mentioned in #517, if you have an entity type configured that does not have a site resolver (or from the UI perspective, preview mode set to None), you get the following error on the canonical entity page:

Call to a member function getSitesForEntity() on null in Drupal\next\Render\MainContent\HtmlRenderer->prepare()

This is because HtmlRenderer assumes getSiteResolver() returns a SiteResolverInterface despite the signature (supported by the UI where you can select none) being:

public function getSiteResolver(): ?SiteResolverInterface;

This PR uses a null object accessor and switches to a falsy (empty array or null) check on the result.

[vercel]

@andrewbelcher is attempting to deploy a commit to the Chapter Three Team on Vercel.

A member of the Team first needs to authorize it.

[JohnAlbin]

Fixed by #481