qml: fix dropping item in playlist view may not open new playlist dialog

This is because itemContainsDrag may be undefined or null, which is observed when the list is empty.

With the chaining operator, the evaluation yields undefined, and then the function can handle the passed undefined index (which is already the case here with acceptDropFunc, where DialogsProvider.playlistsDialog() is called when the passed index is undefined).

Request review @chub.