It works, I have test cases to prove it :)
On a serious note, here's a quote from C# language specification: "If an operand of string concatenation is null, an empty string is substituted."
I also added tests for mentioned numbers and it seems to work fine (-21 returns "Fizz", 35 returns "Buzz", etc.).
Thanks for your feedback!
Interesting, that is a pretty cool feature. Thank you for pulling out the specifications 😊