fix test so that NULLS are always last

databases treat `NULL` differently when ordering, so make them behave
the same by explcitly specifying how to order `NULL`

activerecord/test/cases/relation/field_ordered_values_test.rb

@@ -134,7 +134,7 @@ def test_in_order_of_with_array_values_with_nil
     Book.create!(format: "digital")
 
     order = ["ebook", ["paperback", nil, "digital"], "letter"]
-    books = Book.in_order_of(:format, order).order(format: :desc)
+    books = Book.in_order_of(:format, order).order(Book.arel_table[:format].desc.nulls_last)
     assert_equal(["ebook", "paperback", "digital", nil, "letter"], books.map(&:format))
   end
 end