Fix pushing org-projectile-todo-files to org-agenda-files

Using `(push (org-projectile-todo-files) org-agenda-files)' would occur error shown below when entering org-agenda-mode.
--error begin
Wrong type argument: stringp, ("~/.emacs.d/TODOs.org" ...)
--error end
`(org-projectile-todo-files)' returns a list of files.
 `push' function would make the list a new ELT of `org-agenda-files' rather than merging members into it.
But `org-file-menu-entry' only accept file path string as parameter.

layers/+emacs/org/README.org

@@ -249,7 +249,10 @@ the following snippet. Note that this may have unintended consequences until
 #+BEGIN_SRC emacs-lisp
   (with-eval-after-load 'org-agenda
     (require 'org-projectile)
-    (push (org-projectile-todo-files) org-agenda-files))
+    (mapcar '(lambda (file)
+                   (when (file-exists-p file)
+                     (push file org-agenda-files)))
+            (org-projectile-todo-files)))
 #+END_SRC
 
 ** Org-brain support