Plugin Details Notices: Remove selectedSite as required as it will be null for multisite visualization

[WBerredo]

Proposed Changes

Mark selectedSite as not required as it can be null on multisite visualization.
This should fix the console error on the Plugin Details page for multisite view.

Testing Instructions

• From trunk branch
• Go to a plugins page (/plugins/:plugin-slug). Ex: /plugins/woocommerce
• Check the console, there SHOULD BE an error as stated above
• From this branch
• Go to the same page
• There SHOULD NOT be any errors on the console

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-66551

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-66551&env=jetpack

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[noahtallen]

sounds good 👍

By the way, you don't need to get Calypso's approval for these types of PRs! ;)

[tyxla]

Thanks 👍