Fixes a regression from 1d337249.

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.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=55179