fix another ruby test

Author: Taepper

c_glib/test/test-rank-quantile-options.rb

@@ -42,6 +42,8 @@ def test_add_sort_key
   end
 
   def test_null_placement
+    assert_equal(Arrow::NullPlacement::UNSPECIFIED, @options.null_placement)
+    @options.null_placement = :at_end
     assert_equal(Arrow::NullPlacement::AT_END, @options.null_placement)
     @options.null_placement = :at_start
     assert_equal(Arrow::NullPlacement::AT_START, @options.null_placement)