Problem/Motivation
From #44 we learned that select element adds #empty_option in Select::processSelect. However, if a select element is updated (re-rendered?) on an ajax callback, the #empty_option gets removed.
The original issue reported this behavior of one such instance at /admin/config/development/configuration/single/export.
Proposed resolution
Add #empty_option in a #pre_render callback
Remaining tasks
Provide patch to fix the issue.
Write a test to prevent regression.
Code review and validate the used approach.