Add a test that compares nulls in JavaScript

Author: ledsun

packages/npm-packages/ruby-wasm-wasi/test/unit/test_nil_class.rb

@@ -3,6 +3,7 @@
 
 class TestNilClass < Test::Unit::TestCase
   def test_to_js
+    assert_same JS::Null, nil.to_js
     assert_equal "null", nil.to_js.to_s
   end
 end