Introduce ParametersAreNonnullByDefault and mark nullable parameters as Nullable

[Pazus]

Being a modern library we should mark our base package with @ParametersAreNonnullByDefault annotation and mark all nullable parameters as @Nullable within the code using JSR305.

[pesse]

Sure, go ahead 👍
I didn't know there was such a thing. One of the issues of being 100% self-taught.