Clear sound value when importing database

The file will no longer exist.

lib/settings_page.dart

@@ -98,6 +98,9 @@ class _SettingsPageState extends State<_SettingsPage> {
                                 // ignore: invalid_use_of_protected_member, invalid_use_of_visible_for_testing_member
                                 final migrator = db.createMigrator();
                                 await migrator.createAll();
+                                await db.update(db.settings).write(
+                                    const SettingsCompanion(
+                                        sound: Value(null)));
                                 print('Migrated.');
                               },
                               child: const Text("Import")));