storage: Fix type error returning nothing in dry run case.

zerver/storage.py

@@ -7,7 +7,7 @@ from pipeline.storage import PipelineMixin
 class AddHeaderMixin(object):
     def post_process(self, paths, dry_run=False, **kwargs):
         if dry_run:
-            return
+            return []
 
         with open(settings.STATIC_HEADER_FILE) as header_file:
             header = header_file.read().decode(settings.FILE_CHARSET)