Allow nulls for Method.invoke(null) and Field.get(null)

[vlsi]

The nulls are permitted there to access static fields and methods

[vlsi]

test_plume_lib build fails because it has @SuppressWarnings(nullness:argument.type.incompatible) to suppress field.get(null): https://github.com/plume-lib/options/blob/24be095f55cd9f521b25f5409738e48dd4370481/src/main/java/org/plumelib/options/Options.java#L425

So plume_lib should be adjusted (excessive suppression should be removed) after this PR is merged.

[cpovirk]

This is requested occasionally, as in typetools/checker-framework#1534. The Checker Framework policy is to declare the parameters as non-null to avoid any chance of NullPointerException. There was a comment about this in the old nullness stub. Perhaps it should be carried over to the new file that you're editing here. Additionally, you could consider a PR to provide a separate astub file, as proposed in typetools/checker-framework#3258

[vlsi]

@cpovirk , thanks for looking into this, however, it looks like you are confused.

The PR allows null inputs for instance parameter.
Currently, checkerframework forbids to call static methods via reflection, and it does not really help to track nullability issues :(

That is why I believe it should be generally possible to call field.get(null) to get the value of a static field.

Of course, I can add a stub to my project, however, it is sad that the default mode is "you can't use reflection" :-/

[vlsi]

The comment you mention says "warning", however, in practice the code like field.get(null) raises an error rather than a warning.

[vlsi]

Ah, you want to have non-standard stub. Ok, then I would put that stub into pgjdbc for a while.