title: ActiveRecord serialize sending nil to custom serializer for present attribute
body: <p>I have the following custom serializer:</p>
  
  <pre><code>class ReportCardDataSerializer
    def self.dump(hash)
      hash.to_json
    end
  
    def self.load(json)
      # json.class == NilClass, why????
      hash = (json || {}).with_indifferent_access
    end
  end
  </code></pre>
  
  <p>And the following class with a serialized <code>data</code> attribute with database column set to NOT NULL.</p>
  
  <pre><code>class ReportCardGroup &lt; ActiveRecord::Base
    serialize :data, ReportCardDataSerializer # data is PostgreSQL jsonb column
  end
  </code></pre>
  
  <p>The ReportCardDataSerializer <code>dump</code> method works as expected. But on trying to <code>load</code> ReportCardDataSerializer <code>load</code> method gets sent nil even though the database column is not nil.</p>
  
  <p>Why is this happening?</p>