Fix links without contents in table of contents' entries
* lisp/ox.el (org-export-toc-entry-backend): New function.
* lisp/ox-html.el (org-html--format-toc-headline):
* lisp/ox-md.el (org-md--build-toc):
* lisp/ox-odt.el (org-odt-toc): Use new function.
* lisp/ox-texinfo.el (org-texinfo--sanitize-title):
(org-texinfo--wrap-float): Handle links without contents.
* testing/lisp/test-ox.el (test-org-export/toc-entry-backend): New
test.
Reported-by: ConcreteVitamin <concretevitamin@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-10/msg00235.html>