Address deprecation warning from minitest

Using assert_equal nil will be deprecated in minitest 6. We should
address this deprecation so upgrading will be easier when the time
comes, as well as to keep our test outputs nice and clear.

Deprecation warning:
DEPRECATED: Use assert_nil if expecting nil. This will fail in Minitest 6.

test/presenters/document_collection_presenter_test.rb

@@ -128,7 +128,7 @@ class PresentedDocumentCollection < TestCase
 
         public_updated_at = grouped[0][:metadata][:public_updated_at]
 
-        assert_equal nil, public_updated_at
+        assert_nil nil, public_updated_at
       end
     end
   end