Handle slides for empty presentation

[raulrpearson]

As part of some recent S3 changes, we added a couple of functions to generate/get slide URLs for presentation display. One of these clauses takes a presentation file and calls the 2-arity version of the function:

def get_slide_urls(%PresentationFile{} = presentation) do
  get_slide_urls(presentation.hash, presentation.length)
end

The implicit assumption I made was that a PresentationFile struct would always represent an existing presentation with a non-nil hash and a non-zero length. That's not the case and we've found in production that the 2-arity function is being called as get_slide_urls(nil, 0). This PR adds that clause returning an empty list, which fits what the LiveViews are happy to handle right now.