org-element: Small fix

* lisp/org-element.el (org-element-inline-babel-call-parser): Empty
  arguments return nil instead of the empty string, so as to be on par
  with `org-element-babel-call-parser'.

lisp/org-element.el

@@ -2878,8 +2878,10 @@ Assume point is at the beginning of the babel call."
 		(let ((p (org-element--parse-paired-brackets ?\[)))
 		  (and (org-string-nw-p p)
 		       (replace-regexp-in-string "\n[ \t]*" " " (org-trim p)))))
-	       (arguments (or (org-element--parse-paired-brackets ?\()
-			      (throw :no-object nil)))
+	       (arguments (org-string-nw-p
+			   (or (org-element--parse-paired-brackets ?\()
+			       ;; Parenthesis are mandatory.
+			       (throw :no-object nil))))
 	       (end-header
 		(let ((p (org-element--parse-paired-brackets ?\[)))
 		  (and (org-string-nw-p p)