[BUG] obscure error for invalid :exports

[Max Nikulin]

Hi,

Trying to export (to HTML buffer)
    src_elisp[:exports source]{a}

I have got an obscure error

    org-export-as: Wrong type argument: char-or-string-p, nil

I believe, some meaningful error should be signaled.

I was quite surprised since the following is exported with no error

    #+begin_src elisp :exports source
      a
    #+end_src

So source blocks are more liberal in respect to user errors.

`org-lint' warns

     2 low   Unknown value "source" for header ":exports"

Reproducible with Org main HEAD and 9.5.5

Of course, it should be

        src_elisp[:exports code]{a}

but I was going to test org-element parser on a more tricky input and by 
mistake typed wrong keyword. It took some time to figure that it is not 
a parser error.