org-persist-gc: GC collections without persist file

* lisp/org-persist.el (org-persist-gc): Do not store collections that
lack persist file.  Such collection may emerge, for example, when
writing fails or gets blocked for some reason (in
`org-persist-before-write-hook' or when associated with encrypted
file).

Reported-by: Gustavo Barros <gusbrs.2016@gmail.com>
Link: https://orgmode.org/list/CAM9ALR_F4gjWxOVh_Eq14NOC9nF6g0Cv1jAKb=p4FjPvjqSkWQ@mail.gmail.com

lisp/org-persist.el

@@ -1210,7 +1210,8 @@ Also, remove containers associated with non-existing files."
               (setq expired? t)))
           (if expired?
               (org-persist--gc-persist-file persist-file)
-            (push collection new-index)
+            (when (file-exists-p persist-file)
+              (push collection new-index))
             (dolist (container (plist-get collection :container))
               (dolist (associated-file
                        (org-persist-associated-files:generic