Use both scheduled and the start property to determine start

* ox-taskjuggler.el (org-taskjuggler-get-start): Use both the
scheduled and the start property (if it is in
org-taskjuggler-valid-resource-attributes) to determine the
start of a task or the project.

Thanks to John Hendy for the report and thorough analysis.

contrib/lisp/ox-taskjuggler.el

@@ -458,7 +458,10 @@ ITEM is a headline.  Return value is a string."
 ITEM is a headline.  Return value is a string or nil if ITEM
 doesn't have any start date defined.."
   (let ((scheduled (org-element-property :scheduled item)))
-    (and scheduled (org-timestamp-format scheduled "%Y-%02m-%02d"))))
+    (or
+     (and scheduled (org-timestamp-format scheduled "%Y-%02m-%02d"))
+     (and (member 'start org-taskjuggler-valid-resource-attributes)
+	  (org-element-property :START item)))))
 
 (defun org-taskjuggler-get-end (item)
   "Return end date for task or resource ITEM.