Added null annotations for Field

[DavyLandman]

I've kept these changes in a separate PR, for one thing since I saw that in AccessibleObject::canAccess @cpovirk in aadd76c added a comment about this, so this might be something where you want to be stricter?

As in, you can pass in a null if you're doing reflection to static fields/methods, but I guess it's more common that you intend to use it with an object instance?

[smillst]

Because passing null to these methods may result in a NPE, these parameters cannot be annotated with @Nullable. See https://checkerframework.org/manual/#nullness-jdk-conservative for a more detailed explanation. We would accept these in the stub file https://github.com/typetools/checker-framework/blob/master/checker/src/main/java/org/checkerframework/checker/nullness/sometimes-nullable.astub.

Thanks!