Fix bug caused by blueprint with pages.sort but without pages.template

This PR fixes a problem, when adding `pags.sort: flip` would cause the “add subpage” modal to break.

    pages: true/false

-> worked

    pages:
      sort: flip

-> broke the “add subpage” view


A workarounds was to specify the template explicitly:

    pages:
      sort: flip
      template: foo

-> worked

    pages:
      sort: flip
      template:
           - foo
           - bar

-> worked


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With this PR, the `template` setting defaults to all templates, when using `sort`.

---

There is a conceptual problem remaining: 
What to do when the user intends to flip the sorting of the pages, but wants disable creation of sub pages at the same time?