`null` literals are rejected, so any null check stops verification

[CatarinaGamboa]

Description

Any null literal (x = null, x == null, new IIOImage(img, null, null)) produces Error: Null literals are not supported, and the rest of the file is not verified. Null checks are everywhere in real code.

Minimal reproducer

public class Repro {
    public static void main(String[] args) {
        String name = null;
        if (name == null) {
            System.out.println("no name");
        }
    }
}

Expected

At least no error: until nulls are supported in refinements, treat a null literal (and comparisons with it) as carrying no information, the same as an unknown value.

Actual

Running LiquidJava on: <repro dir>

Error: Null literals are not supported
2 |     public static void main(String[] args) {
3 |         String name = null;
4 |         if (name == null) {
  |                     ^^^^
5 |             System.out.println("no name");
6 |         }

Reproduced on main at 8816186 (liquidjava-verifier 0.0.35), and on 0.0.33.

Context

Found while turning real open-source Java code into study examples for the error-message study: real code hits this constantly, so the examples have to be rewritten around it or cannot be verified. It affects 6 of 12 examples (blocks one, and forces rewrites in five, e.g. X s = null; try { s = new X(); ... } finally { if (s != null) s.close(); }).