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/Set.node.ts

@@ -12,13 +12,14 @@ export class Set extends VersionedNodeType {
 			icon: 'fa:pen',
 			group: ['input'],
 			description: 'Add or edit fields on an input item and optionally remove other fields',
-			defaultVersion: 3,
+			defaultVersion: 3.1,
 		};
 
 		const nodeVersions: IVersionedNodeType['nodeVersions'] = {
 			1: new SetV1(baseDescription),
 			2: new SetV1(baseDescription),
 			3: new SetV2(baseDescription),
+			3.1: new SetV2(baseDescription),
 		};
 
 		super(nodeVersions, baseDescription);