Fix NPE When Incrementing Fields

[rcosta358]

Description

Fixes a null pointer exception when applying unary update operators such as field++ to class fields.
This fix also applies to other unary field updates such as field--, ++field, and --field.

Example

public class Test {
    private int field;

    public void increment() {
        field++; // crash
    }
}

Related Issue

None.

Type of change

• Bug fix
• New feature
• Documentation update
• Code refactoring

Checklist

• Added/updated tests under liquidjava-example/src/main/java/testSuite/ (Correct* / Error*)
• mvn test passes locally
• Updated docs/README if behavior or API changed