Generated CSharp code is not compatible with downlevel language versions

[aschultz]

The code generated by LottieGen in csharp mode uses language features like is not null that are only available in C# 9.0 and above. UWP applications are still stuck on older versions of C# (7.3 I think?), so the generated code is unusable. I'm currently running into this while developing Windows support in lottie-react-native.

Please replace these constructs with more compatible ones. We don't need syntactic sugar in generated code if it's going to cause compatibility problems. The core language was plenty capable even back to C# 3 or 4.