org-bibtex: Mark filename argument as required

* lisp/org-bibtex.el (org-bibtex): Mark FILENAME argument as required
  because, when called from Lisp, the function does not provide a
  default file name when FILENAME is nil, leading to a type error.

lisp/org-bibtex.el

@@ -549,7 +549,7 @@ With optional argument OPTIONAL, also prompt for optional fields."
 
 
 ;;; Bibtex <-> Org-mode headline translation functions
-(defun org-bibtex (&optional filename)
+(defun org-bibtex (filename)
   "Export each headline in the current file to a bibtex entry.
 Headlines are exported using `org-bibtex-headline'."
   (interactive