Always requests confirmation for changing music folder

Noise always requests confirmation when changing music folder and suggests that existing data will be destroyed even when there's no data to destroy. A screenshot of the prompt is attached.

The bug comes from src/LibraryWindow.vala line 1178 - it checks whether the linked list of playlists is empty, but it's never empty because Noise always has two built-in playlists "Queue" and "History".

It should probably use a smarter check, like the one in the confirmation dialog itself that makes the "Export playlists" button insensitive.