[O] org-mode `org-capture-templates' %(sexp) must return string restriction

[Oleh]

Hi all,

Is this restriction really necessary?
This results in code like:

(setq
 org-capture-templates
 '(("e" "elisp todo"
    entry
    (file+headline (concat org.d "gtd.org") "Tasks")
    "* TODO %(progn (org-set-tags-to \"ELISP\") \"\")%^{Brief
Description}\n  Added: %U  %i\n  %?\n")))

I think it should be fine to just return nil. That would be equivalent
to the empty string.
Here's the same code if returning nil would be acceptable:

(setq
 org-capture-templates
 '(("e" "elisp todo"
    entry
    (file+headline (concat org.d "gtd.org") "Tasks")
    "* TODO %(org-set-tags-to \"ELISP\")%^{Brief Description}\n
Added: %U  %i\n  %?\n")))

Any opinions on this?

regards,
Oleh