PATCH: services: correct behaviour of setting service's description

Created an attachment (id=7799)
Patch

This patch corrects the behaviour of the SERVICE_CONFIG_DESCRIPTION case in RChangeServiceConfig2W().

The correct behaviour is that, "if [the lpDescription pointer] is NULL, the description remains unchanged. If this value is an empty string (""), the current description is deleted." (SERVICE_DESCRIPTION structure description from MSDN).