Fix erroring fog test

The Fog mock set-up for sending files to S3 raises an exception if the
data it is passed is nil. This can be fixed by updating this stub to
return an empty string.

test/unit/workers/document_list_export_worker_test.rb

@@ -14,7 +14,7 @@ class DocumentListExportWorkerTest < ActiveSupport::TestCase
 
   test "instantiates an EditionFilter with passed options converted to symbols" do
     Admin::EditionFilter.expects(:new).with(Edition, @user, state: "draft", include_unpublishing: true, include_locked_documents: false)
-    @worker.stubs(:generate_csv)
+    @worker.stubs(generate_csv: "")
     @worker.stubs(:send_mail)
     @worker.perform({ "state" => "draft" }, @user.id)
   end