[Fix clojure-emacs/cider#1234] project-dir is nil if not in a project

clojure-mode.el

@@ -982,16 +982,18 @@ nil."
 (defun clojure-project-dir (&optional dir-name)
   "Return the absolute path to the project's root directory.
 
-Use `default-directory' if DIR-NAME is nil."
+Use `default-directory' if DIR-NAME is nil.
+Return nil if not inside a project."
   (let ((dir-name (or dir-name default-directory)))
     (let ((lein-project-dir (locate-dominating-file dir-name "project.clj"))
           (boot-project-dir (locate-dominating-file dir-name "build.boot")))
-      (file-truename
-       (cond ((not lein-project-dir) boot-project-dir)
-             ((not boot-project-dir) lein-project-dir)
-             (t (if (file-in-directory-p lein-project-dir boot-project-dir)
-                    lein-project-dir
-                  boot-project-dir)))))))
+      (when (or lein-project-dir boot-project-dir)
+        (file-truename
+         (cond ((not lein-project-dir) boot-project-dir)
+               ((not boot-project-dir) lein-project-dir)
+               (t (if (file-in-directory-p lein-project-dir boot-project-dir)
+                      lein-project-dir
+                    boot-project-dir))))))))
 
 (defun clojure-project-relative-path (path)
   "Denormalize PATH by making it relative to the project root."