Metadata robots noindex causes build error

[chernodub]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

Faced error when tried to build the application with metadata: [{name: 'robots', content: 'noindex'}] set in docusaurus.config.js. This is a recommended way of hiding the website from search crawlers. I was also able to avoid the issue by using an alternative way of applying noindex in metadata.

Reproducible demo

https://github.com/chernodub/docusaurus-build-issue-repro

Steps to reproduce

With attached demo:

1. Clone the repo
2. npm install
3. npm run build

With clean docusaurus build:

1. npx create-docusaurus@latest my-website classic
2. cd my-website
3. Add metadata: [{name: 'robots', content: 'noindex'}] to docusaurus.config.js
4. npm run build

Expected behavior

npm run build finishes without errors

Actual behavior

npm run build fails with an error

node:events:505
      throw er; // Unhandled 'error' event
      ^

EmptySitemap: You ended the stream without writing anything.
Emitted 'error' event on SitemapStream instance at:
    at emitErrorNT (node:internal/streams/destroy:157:8)
    at emitErrorCloseNT (node:internal/streams/destroy:122:3)
    at processTicksAndRejections (node:internal/process/task_queues:83:21)

Your environment

• Docusaurus version used: 2.0.0-beta.22
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Node.js 16.6.0
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): macOS Monterey 12.4

Self-service

• I'd be willing to fix this bug myself.