[O] Conditionally formatting org-html-postamble-format

[Kaushal Modi]

Hi,

I have the org-html-postamble-format set to the below:

(setq org-html-postamble-format

                  `(("en"

                     ,(concat "Exported using "

                              ;; "%c" is replaced with `org-html-creator-string'

                              ;; Emacs <VERSION> (Org mode <VERSION>)

                              "<div style=\"display: inline\" class=\"creator\">"

                              "%c</div> "

                              "by %a. — "

                              "<div style=\"display: inline\" class=\"date\">"

                              "%d</div>"))))

It works great except for the cases where I have set a document author to nothing using 

#+AUTHOR:

What would be the best way to set the postamble so that the "by %a" portion does not get printed if the %a value is "".

Right now, if the author is nil, the postamble gets exported as:

Exported using 

Emacs 25.0.50.1 (Org mode 8.3.3)

 by . — 

Jan 26 2016, Tue