Fix spacing between empty state message and 'Add new timeline event' button

Context

There is a bit of additional extra space that's being added above the Add new timeline event button in its empty state form.

Prior discussions: #358637 (comment 1024556024)

Proposal

We should change the spacing from 32 to 24 px depending on whether it's the empty state or not. Users won't often see the empty state, as some timeline events are automatically added. However, it's possible to get back to the empty state by deleting the system timeline events.

Intended spacing (24px)	Current spacing (32px)