gschema: Fix invalid default value for previous-queue

Author: ryonakano

data/music.gschema.xml

@@ -13,7 +13,7 @@
       <description>An index representing the repeat mode</description>
     </key>
     <key type="as" name="previous-queue">
-      <default>['']</default>
+      <default>[]</default>
       <summary>The queue from last session to restore</summary>
       <description>An array of strings representing the files played last</description>
     </key>