Consider null descriptor as valid argument for create/destroy cases.

Change-Id: I6b25677fe35e2ad4a7d678ad3e5eac290681f6e9
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/187498
Reviewed-by: Sebastian Zarnekow <sebastian.zarnekow@gmail.com>