Annotate all packages with Spring Frameworks @NonNullApi and @NonNullFields. Add Spring's @Nullable to methods, parameters and fields that take or produce null values. Adapted using code to make sure the IDE can evaluate the null flow properly. Fix Javadoc in places where an invalid null handling policy was advertised. Strengthened null requirements for types that expose null-instances.
Original Pull Request: #28
83057f1288