config: fix bug in default backup storage manager config.

Fix uploads failing in the default configuration because the backup
storage manager didn't initialize the null storage manager with the
required `base_url` and `base_dir` options.

app/logical/storage_manager/null.rb

@@ -1,4 +1,8 @@
 class StorageManager::Null < StorageManager
+  def initialize(base_url: "/", base_dir: "/")
+    super
+  end
+
   def store(io, path)
     # no-op
   end