search-in-workspace: correct scrollbar display when no results

[smart-bo]

What it does

fix #10289
Correct unexpect scrollbar display when no results are present.
Make sure the change don't effect issue #9203

How to test

• open the search-in-workspace widget
• confirm that no search term is present
• open the replace field (arrow)
• scrolling the empty area
• confirm no scrollbar show up
• drag and drop sections between views, confirm it works well in different Theia versions

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The changes unfortunately cause a major regression when searching, the results are never visible: