Fix crash in task.imported_items (#6326)

Fixes #6291

I think it does not hurt if the imported_items() method returned an
empty list instead of an throwing an exception if there is nothing in
it!

beets/importer/tasks.py

@@ -253,7 +253,7 @@ class ImportTask(BaseImportTask):
         ):
             return self.match.items
         else:
-            assert False
+            return []
 
     def apply_metadata(self):
         """Copy metadata from match info to the items."""