Windows: Fix return value of QDesktopServices::storageLocation(DataLocation).

Return empty string if SHGetSpecialFolderPath() fails for DataLocation, ie,
the directory does not exist.

Change-Id: I2abf41174b904fe36e587894832c0f3f4947dc1e
Task-number: QTBUG-43092
Reviewed-by: David Faure <david.faure@kdab.com>