Added support for nullable reference types

We have updated all our APIs by adding support for nullable reference types. This is a C# feature that helps you minimize NullReferenceException occurrence in runtime by performing a null-state analysis. The feature is disabled by default for existing projects, but it is enabled for new .NET 6 projects. To enable it manually, just add the following line to your .csproj file: <Nullable>enable</Nullable>. This will enable the nullable annotation context and start showing warnings when your code may dereference null. For detailed information on nullable reference types and null-state analysis, please refer to Microsoft documentation.