Apply jspecify nullability annotations for better Java & Kotlin experience

[MukjepScarlet]

Problem solved by the feature

Kotlin compiler (so does Java hint) doesn't know the parameter's nullability.

Feature description

Add Nullable, Nonnull and so on.

Alternatives / workarounds

Libraries like Spring has such support.

[eamonnmcmanus]

Yes, we should do this, especially because the team at Google that is responsible for Gson also did a lot of the work behind JSpecify. One caution is that adding the annotations is likely to make it so that some Kotlin client code that used to compile no longer does. In the absence of nullness annotations, the Kotlin compiler doesn't enforce compile-time nullness checks on parameters and return types of Java methods.

[MukjepScarlet]

I think it's worth to do that. If you want I can start to do it. @eamonnmcmanus