fix(Google Sheets Node): Prevent erroring when fetching mapping columns (#7972)

## Summary
https://n8nio.sentry.io/issues/4709551520

Follow-up to: https://github.com/n8n-io/n8n/pull/7957

...

#### How to test the change:
1. ...


## Issues fixed
Include links to Github issue or Community forum post or **Linear
ticket**:
> Important in order to close automatically and provide context to
reviewers

...


## Review / Merge checklist
- [ ] PR title and summary are descriptive. **Remember, the title
automatically goes into the changelog. Use `(no-changelog)` otherwise.**
([conventions](https://github.com/n8n-io/n8n/blob/master/.github/pull_request_title_conventions.md))
- [ ] [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up
ticket created.
- [ ] Tests included.
> A bug is not considered fixed, unless a test is added to prevent it
from happening again. A feature is not complete without tests.
  >
> *(internal)* You can use Slack commands to trigger [e2e
tests](https://www.notion.so/n8n/How-to-use-Test-Instances-d65f49dfc51f441ea44367fb6f67eb0a?pvs=4#a39f9e5ba64a48b58a71d81c837e8227)
or [deploy test
instance](https://www.notion.so/n8n/How-to-use-Test-Instances-d65f49dfc51f441ea44367fb6f67eb0a?pvs=4#f6a177d32bde4b57ae2da0b8e454bfce)
or [deploy early access version on
Cloud](https://www.notion.so/n8n/Cloudbot-3dbe779836004972b7057bc989526998?pvs=4#fef2d36ab02247e1a0f65a74f6fb534e).

packages/nodes-base/nodes/Google/Sheet/v2/methods/resourceMapping.ts

@@ -11,7 +11,12 @@ import { getSpreadsheetId } from '../helpers/GoogleSheets.utils';
 export async function getMappingColumns(
 	this: ILoadOptionsFunctions,
 ): Promise<ResourceMapperFields> {
-	const { mode, value } = this.getNodeParameter('documentId', 0) as IDataObject;
+	const documentId = this.getNodeParameter('documentId', 0) as IDataObject | null;
+
+	if (!documentId) return { fields: [] };
+
+	const { mode, value } = documentId;
+
 	const spreadsheetId = getSpreadsheetId(this.getNode(), mode as ResourceLocator, value as string);
 
 	const sheet = new GoogleSheet(spreadsheetId, this);