Describe the bug
Enabling site kit plugin breaks the AMP version of the site. It just showing 49 years ago, article tags and categories.

To Reproduce
Steps to reproduce the behavior:

1. Install Site Kit plugin and configure it
2. Install AMP plugin By AMP Project Contributors
3. Try accessing AMP page of the site
4. See error

Screenshot

System Information (please complete the following information):

• PHP Version: 7.2.19
• Browser: chrome
• Plugin Version 1.0.0-beta.1

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Changelog entry

• Fix PHP notices being thrown on AMP reader mode when Analytics or Tag Manager snippet is not inserted.

Acceptance criteria

• Activate the AMP plugin and configure it to use Reader mode in its settings screen.
• Configure Site Kit's Analytics module. In Site Kit > Settings, ensure the snippet is not inserted.
• Open a single post in the frontend and click on "AMP" in the admin bar to view its AMP version.
• Verify that no PHP notices are being thrown and the page renders correctly.
• Start setup of Site Kit's Tag Manager module, but then immediately bail out (e.g. by going to another WordPress page), so that the setup is flagged as incomplete.
• Open a single post in the frontend and click on "AMP" in the admin bar to view its AMP version.
• Verify that no PHP notices are being thrown and the page renders correctly.