Fix blank slides issue in Reveal slideshow pdf export

Applied the bugfix made on the original index.html from
reveal.js project into the template :

- https://github.com/hakimel/reveal.js/commit/08fb6cb2f96a6d97aaa21c35a925ebb59b0b0070
Merge pull request #5671 from ncornette/master

Fix blank slides issue in Reveal slideshow pdf export