fix(core): Fix error on missing paired item data (#7399)

<img width="1015" alt="Capture 2023-10-11 at 10 07 02@2x"
src="https://github.com/n8n-io/n8n/assets/44588767/0305f62d-0a7b-4ce7-b78f-a8d8454e2532">

packages/workflow/src/WorkflowDataProxy.ts

@@ -699,7 +699,7 @@ export class WorkflowDataProxy {
 						' <a target="_blank" href="https://docs.n8n.io/data/data-mapping/data-item-linking/item-linking-errors/">More info</a>';
 
 					context.description += moreInfoLink;
-					context.descriptionTemplate += moreInfoLink;
+					if (context.descriptionTemplate) context.descriptionTemplate += moreInfoLink;
 				}
 			}