test(instance) Use assert_nil instead of assert_equal nil, ….

Minitest emits a warning. This patch fixes that.

Author: Hywan

tests/instance_test.rb

@@ -104,7 +104,7 @@ def test_call_string
   end
 
   def test_call_void
-    assert_equal nil, Wasmer::Instance.new(self.bytes).exports.void
+    assert_nil Wasmer::Instance.new(self.bytes).exports.void
   end
 
   def test_exports