fix(Set Node): Do not stringify null and undefined (#7313)

Github issue / Community forum post (link here to close automatically):
https://community.n8n.io/t/null-and-undefined-in-string-fields/31060/1

packages/nodes-base/nodes/Set/v2/SetV2.node.ts

@@ -21,7 +21,7 @@ const versionDescription: INodeTypeDescription = {
 	name: 'set',
 	icon: 'fa:pen',
 	group: ['input'],
-	version: 3,
+	version: [3, 3.1],
 	description: 'Change the structure of your items',
 	subtitle: '={{$parameter["mode"]}}',
 	defaults: {