I added nullable annotation to classes A, B, C.

https://github.com/pCYSl5EDgo/MessagePack-CSharp/blob/test_nullable/sandbox/TestData2/A.cs
https://github.com/pCYSl5EDgo/MessagePack-CSharp/blob/test_nullable/sandbox/TestData2/B.cs
https://github.com/pCYSl5EDgo/MessagePack-CSharp/blob/test_nullable/sandbox/TestData2/C.cs

B?Formatter and C?Formatter appeared.

https://github.com/pCYSl5EDgo/MessagePack-CSharp/blob/test_nullable/sandbox/TestData2/Generated.cs