The following example calls ‘mailcap-viewer-passes-test’ four times.
For the first, second and third call the VIEWER-INFO does not contain a
test; for the third the test is nil.  The results show that the first
and second call correctly return t; the third with (test . nil)
correctly returns nil and adds (nil nil) to the cache; the fourth, then,
incorrectly returns nil.

     #+HEADER: :results table
     #+begin_src emacs-lisp
       (let (mailcap-viewer-test-cache
             (viewer-infos (list (list (cons 'viewer "viewer-w/o-test"))
                                 (list (cons 'viewer "viewer-w/o-test"))
                                 (list (cons 'viewer "viewer-w/-test-nil")
                                       (cons 'test nil))
                                 (list (cons 'viewer "viewer-w/o-test")))))
         (cl-list* (list "Index" "Viewer" "Passes Test" "Cache after Call")
                   'hline
                   (cl-loop for i = 1 then (1+ i)
                            for vi in viewer-infos
                            collect (list (format "%i." i)
                                          (alist-get 'viewer vi)
                                          (mailcap-viewer-passes-test vi nil)
                                          mailcap-viewer-test-cache))))
     #+end_src

     #+RESULTS:
     | Index | Viewer             | Passes Test | Cache after Call |
     |-------+--------------------+-------------+------------------|
     |    1. | viewer-w/o-test    | t           | nil              |
     |    2. | viewer-w/o-test    | t           | nil              |
     |    3. | viewer-w/-test-nil | nil         | ((nil nil))      |
     |    4. | viewer-w/o-test    | nil         | ((nil nil))      |


-- 
Felix Dietrich