Fix the type of Hatenablog::Client#next_feed
Fix the type of Hatenablog::Client#next_feed

> lib/hatenablog/client.rb:70:6: [error] The method cannot return a value of type `::Hatenablog::Feed` because declared as type `nil`
> │   ::Hatenablog::Feed <: nil
> │
> │ Diagnostic ID: Ruby::ReturnTypeMismatch
> │
> └       return Feed.load_xml(get_collection(collection_uri).body) if feed.nil?
>         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

kymmt90/hatenablog,kymmt90/hatenablog