[EuiPopover] Handle initialFocus

[thompsongl]

Summary

Fixes #4767, where it was discovered that EuiPopover would exit early from its method to set initial focus, causing the initialFocus prop to be disregarded.

The fix here is to ensure that focus has been intentionally set, not just that any element has focus.
false is now an option for intialFocus to allow consumers to opt-out of initial auto-focus while maintaining focus trapping.

Wasn't able to write a test that actually worked.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile

• Checked in Chrome, Safari, Edge, and Firefox
• Props have proper autodocs and playground toggles
• Added documentation
• Checked Code Sandbox works for the any docs examples

- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4768/

[chandlerprall]

Changes LGTM! Verified the popfirst change works as expected in the form layouts' inline popover example.