ob-java: Fix previous commit

* lisp/ob-java.el (org-babel-execute:java): Restore inspection of
:dir's value to decide whether to run from temporary directory.

My compatibility fix in the previous commit incorrectly changed the
documented and intended behavior for (:dir . nil).

Reported-by: Ian Martins <ianxm@jhu.edu>

lisp/ob-java.el

@@ -172,7 +172,7 @@ replaced in this string.")
           (or (cdr (assq :java params))
               org-babel-java-command))
          ;; if true, run from babel temp directory
-         (run-from-temp (not (assq :dir params)))
+         (run-from-temp (not (cdr (assq :dir params))))
          ;; class and package
          (fullclassname (or (cdr (assq :classname params))
                             (org-babel-java-find-classname body)))