Fix for empty figcaptions that works properly in both saxon and xsltproc.
Fix for empty figcaptions that works properly in both saxon and xsltproc.

gitenberg-dev/HTMLBook,mushroomgithub/HTMLBook,wastemobile/HTMLBook,macmillanpublishers/HTMLBook,macmillanpublishers/HTMLBook,oreillymedia/HTMLBook,gitenberg-dev/HTMLBook,wastemobile/HTMLBook,mushroomgithub/HTMLBook,gitenberg-dev/HTMLBook,macmillanpublishers/HTMLBook,mushroomgithub/HTMLBook,oreillymedia/HTMLBook,oreillymedia/HTMLBook