ob-clojure-cli-command: Fix :type declaration

* lisp/ob-clojure.el (ob-clojure-cli-command): Allow nil value.  It
can happen, even though it will yield error (which is a different
issue).  But let's follow other defcustoms in the file.

Reported-by: Mattias Engdegård <mattias.engdegard@gmail.com>
Link: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=71566

lisp/ob-clojure.el

@@ -130,7 +130,7 @@
 (defcustom ob-clojure-cli-command (when-let (cmd (executable-find "clojure"))
                                     (concat cmd " -M"))
   "Clojure CLI command used by the Clojure `clojure-cli' backend."
-  :type 'string
+  :type '(choice string (const nil))
   :group 'org-babel
   :package-version '(Org . "9.7"))