Extensions are sorted by categories.
EXT: t3skin does not have a default category.

Change-Id: I525c0d22a32441d63c5b8561a009ecf43bd58d56
Fixes: #38510
Releases: 6.0
Reviewed-on: http://review.typo3.org/12487
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn