org-element--cache-process-request: Fix phase 2 with nil parent field

* lisp/org-element.el (org-element--cache-process-request): Fix
potential error raised when merging phase 2 requests with current
request having nil parent field.

Should help with https://list.orgmode.org/PAXPR08MB664034C85DDC73D21AD1CB0FA39E9@PAXPR08MB6640.eurprd08.prod.outlook.com/T/#t

lisp/org-element.el

@@ -6155,6 +6155,7 @@ completing the request."
                 (let ((next-request (nth 1 org-element--cache-sync-requests)))
                   (unless (and (org-element-property :cached (org-element--request-parent next-request))
                                (org-element-property :begin (org-element--request-parent next-request))
+                               parent
                                (> (org-element-property :begin (org-element--request-parent next-request))
                                   (org-element-property :begin parent)))
                     (setf (org-element--request-parent next-request) parent)))