Don't arbitrarily pick an orphan as the home page of a space which doesn't have any and provide one instead

Currently, if a space doesn't have a homePage property, an orphan is picked arbitrarily as the home page, in the hope we work around corrupted exports.

But not having a home page is a fully valid and supported situation in Confluence.

We need to stop doing this, and instead provide a default home page (so confluence space resolvers keep working).

We will provide a default body content that simply consists in a page tree of the space.