ob-core: Fix "wrong-type-argument stringp nil"

* lisp/ob-core.el (org-babel-merge-params): Tolerate empty :exports
  parameter.

Reported-by: Rainer M Krug <Rainer@krugs.de>
<http://permalink.gmane.org/gmane.emacs.orgmode/102005>

lisp/ob-core.el

@@ -2551,7 +2551,8 @@ parameters when merging lists."
 	       (setq params (cons pair (assq-delete-all (car pair) params)))))
 	    (:exports
 	     (setq exports (funcall e-merge exports-exclusive-groups
-				    exports (split-string (cdr pair)))))
+				    exports
+				    (split-string (or (cdr pair) "")))))
 	    (:tangle ;; take the latest -- always overwrite
 	     (setq tangle (or (list (cdr pair)) tangle)))
 	    (:noweb