Avoid Liquid failure when no pages with titles

Fix issue #1085

The user's config specified collections (incorrectly). Trying to build the site resulted in Jekyll failing due to a Liquid error. The error report did not suggest the cause of the error.

Liquid fails with division by 0 when title_pages_size is 0. This fix guards that code by checking that title_pages is non-empty.

To test:

1.  Specify a Jekyll collection with no pages, and specify it as a JTD collection.
2. Build the site.
3. Check that the specified collection has no nav links to pages.