Resolves DOTCOM-14138 Staging Sites Redesign: Add staging site button appears briefly when reloading

Proposed Changes

• Added a loading condition to showAddStagingButton
• Now the button only shows when not loading AND there are no staging sites

Why are these changes being made?

When reloading a page that has a staging site, the Add staging site + button has been appearing briefly.

Testing Instructions

• Apply these changes or open the Calypso Live link below
• Navigate to a site with a staging site and refresh the page
• Check in the header that the Add staging site + button is not rendered at all
• Navigate to a site that doesn't have a staging site and refresh the page
• Check in the header that the Add staging site + button is rendered after the page is initially loaded

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 tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• 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 UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• 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-aUh-p2)?