[O] [org-element, FR] make secondary-string midnight compatible

[Rasmus]

Hi,

As is evident from the git log, I have made the following mistake at least
twice:

   (org-element-parse-secondary-string nil restrictions)

I wish this would just return nil instead of an error.  In particular, in
ox files you have to do

   (org-element-parse-secondary-string (or whatever "") restrictions)

Which is a bit noisy.

For me the confusion arise because (org-export-data nil info) does not
throw an error.

Note, I have no idea if the error behavior is needed elsewhere, so it
don't know how feasible this request is or if it's heretic for that
matter.

—Rasmus

-- 
The right to be left alone is a human right