org-persist.el: Do not write global variables on buffer kill

* lisp/org-persist.el (org-persist-write-all): Prevent writing global
variables when BUFFER arg is provided.
(org-persist-gc): Handle case when :persist-file is set to nil in the
index.

Fixes https://list.orgmode.org/DB9PR08MB66336CDF0F43DE3DB1D08649A3869@DB9PR08MB6633.eurprd08.prod.outlook.com/T/#m40fcc5ea8ba0ef52518a58713cf9ad3d1fffa33d

lisp/org-persist.el

@@ -209,11 +209,12 @@ When BUFFER is `all', unregister VAR in all buffers."
   "Save all the persistent data."
   (unless (and buffer (not (buffer-file-name buffer)))
     (dolist (index org-persist--index)
-      (when (or (not (plist-get index :path))
-                (and (get-file-buffer (plist-get index :path))
-                     (or (not buffer)
-                         (equal (buffer-file-name buffer)
-                                (plist-get index :path)))))
+      (when (or (and (not (plist-get index :path))
+                     (not buffer))
+                (and (plist-get index :path)
+                     (get-file-buffer (plist-get index :path))
+                     (equal (buffer-file-name buffer)
+                            (plist-get index :path))))
         (org-persist-write (plist-get index :variable)
                 (when (plist-get index :path)
                   (get-file-buffer (plist-get index :path))))))))
@@ -272,9 +273,10 @@ When BUFFER is `all', unregister VAR in all buffers."
   (let (new-index)
     (dolist (index org-persist--index)
       (let ((file (plist-get index :path))
-            (persist-file (org-file-name-concat
-                           org-persist-directory
-                           (plist-get index :persist-file))))
+            (persist-file (when (plist-get index :persist-file)
+                            (org-file-name-concat
+                             org-persist-directory
+                             (plist-get index :persist-file)))))
         (when (and file persist-file)
           (if (file-exists-p file)
               (push index new-index)