gschema: Fix invalid default value for previous-queue

[ryonakano]

Fixes #829

The default value of previous-queue is a string array with one element that has a blank URI. This results last_session_uri.length is not 0 but 1, causing last_session_files has one File that has a blank URI.

music/src/PlaybackManager.vala

Lines 527 to 541 in c2fc713

	public void restore_queue () {
	// Restoring the queue overwrites the last played. So we need to retrieve it before taking care of the queue
	var uri_last_played = settings.get_string ("uri-last-played");
	var file_last_played = File.new_for_uri (uri_last_played);
	var last_session_uri = settings.get_strv ("previous-queue");
	var last_session_files = new File[last_session_uri.length];
	for (var i = 0; i < last_session_uri.length; i++) {
	var uri = last_session_uri[i];
	var file = File.new_for_uri (uri);
	last_session_files[i] = file;
	}
	var files_to_play = Application.loop_through_files (last_session_files);

causing the terminal warning here

music/src/Application.vala

Line 141 in c2fc713

if (FileUtils.test (file_path, FileTest.IS_DIR)) {

and causing the toast popup here

music/src/PlaybackManager.vala

Lines 104 to 130 in c2fc713

	public void queue_files (File[] files) {
	discoverer.start ();
	int invalids = 0;
	foreach (unowned var file in files) {
	if (file.query_exists () && "audio" in ContentType.guess (file.get_uri (), null, null)) {
	var audio_object = new AudioObject (file.get_uri ());
	string? basename = file.get_basename ();
	if (basename != null) {
	audio_object.title = basename;
	} else {
	audio_object.title = audio_object.uri;
	}
	discoverer.discover_uri_async (audio_object.uri);
	queue_liststore.append (audio_object);
	} else {
	invalids++;
	continue;
	}
	}
	if (invalids > 0) {
	invalids_found (invalids);
	}