Add nullable convert method

[tenhobi]

The convert method returns T, and when a source is null, it returns the whenSourceIsNull value if it is present or an Exception.

While it might be the use case we want, some people might want a less strict method that could return null when the source is null, etc. To make it type-safe, the original convert method should still return T while the new method tryConvert should be created with return type T?.

With spec

Target convert<Source, Target>(Source? model);
Target? tryConvert<Source, Target>(Source? model);