[CollapsedItemActions] Account for null ref object

[thompsongl]

Summary

The registerPopoverDiv callback was not accounting for null as a possible return value.
In the scenerio where popoverDiv is null and this.popoverDiv is not defined, we would attempt to add an event listener to null, which results in a type error.

Solution is to update the type and logic to account for null

Checklist

• Checked in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• Checked Code Sandbox works for any docs examples
• Checked for breaking changes and labeled appropriately
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6145/

[chandlerprall]

LGTM, tested with the reverted docs change without error

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6145/