ox-koma-letter: Handle empty hyperref LaTeX template

* contrib/lisp/ox-koma-letter.el (org-koma-letter-template): Value
for :latex-hyperref-template can be nil.  Handle this case.

Reported-by: Charles Millar <millarc@verizon.net>
<http://lists.gnu.org/r/emacs-orgmode/2020-04/msg00025.html>

contrib/lisp/ox-koma-letter.el

@@ -748,7 +748,8 @@ holding export options."
                 (if (symbolp with-subject) with-subject
                   (mapconcat #'symbol-name with-subject ","))))
       ;; Hyperref.
-      (format-spec hyperref-template spec)
+      (and (stringp hyperref-template)
+	   (format-spec hyperref-template spec))
       ;; Document start.
       "\\begin{document}\n\n"
       ;; Subject and title.