Nullability Annotations to Java Classes - Annotations & Processor - Annotations

[ParaskP7]

Parent #2827

This issue is about adding missing nullability annotations (@Nullable & @NonNull) to as many Java-related Annotations classes as possible, found within the fluxc-annotations module, including fields, arguments, parameters, and return values.

FYI: As a reference, an example of such class is Action.java.