[Alert details page] Showing empty state when there is no related alerts

[maryam-saeidi]

Figma design

Summary

Currently, we don't show the related alerts tab when there are no related alerts. We need to discuss whether we always want to show this tab with an empty state or if the current logic is acceptable. If we decide to show an empty state, we need a design for it.

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)