[EuiCollapsibleNavBeta] Fix accordion open animation bug

[cee-chen]

Summary

⚠️ Quick note about this bug: it only occurs when toggling an EuiAccordion that's the last item in the items array.

Before	After
Note the "Get started" item jumping on every open	"Get started" item does not jump

QA

• Go to http://eui.elastic.co/pr_7317/?path=/story/euicollapsiblenavbeta--kibana-example
• Click the Observability accordion arrow
• Scroll down to AIOps and click that accordion arrow
• Confirm that repeatedly opening and closing that toggle does not produce any weird visual bugs or jumping

General checklist

• Browser QA
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
    - [ ] Checked for accessibility including keyboard-only and screenreader modes
    - [ ] Checked in both light and dark modes
• Docs site QA - N/A, beta component
• Code quality checklist - N/A, CSS change
• Release checklist - N/A, beta component
• Designer checklist - N/A, beta component

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7317/
• Storybook: https://eui.elastic.co/pr_7317/storybook

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: a151815