* lisp/ox-latex.el (org-latex-example-block, org-latex-src-block):
Ignore element if it's empty. This fixes error "apply: Wrong number
of arguments: max, 0".
* lisp/ox-beamer.el (org-beamer-template): Provide an error when LaTeX
class is invalid.
* lisp/ox-latex.el (org-latex-template): Provide an error when LaTeX
class is invalid.
Patch suggested by Sébastien Vauban