Fixes a regression from 1d337249f16ed61163315019f5dbbe4db3386390.
Prior to the commit, we were solving this case in a different way. If `IDropTarget::DragOver()` returned `DROPEFFECT_NONE`, `X11DRV_XDND_DROPEFFECTToXdndAction()` would convert it to `XdndActionCopy`.