[Bug] DynFields.AlwaysNull throws NullPointerException when bound to a target

[LuciferYang]

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Describe the bug

DynFields.Builder.build(target) and buildChecked(target) throw NullPointerException instead of returning a field bound to null, when defaultAlwaysNull() is set and no candidate field is found.

DynFields.AlwaysNull is a singleton constructed with a null backing Field (super(null, "AlwaysNull")). It does not override bind(Object), so the call falls through to UnboundField.bind, which exempts the sentinel from the static-field check and then dereferences the null field on the next check:

public BoundField<T> bind(Object target) {
  if (isStatic() && this != AlwaysNull.INSTANCE) {
    throw new IllegalStateException("Cannot bind static field " + name);
  }
  if (!field.getDeclaringClass().isAssignableFrom(target.getClass())) {   // NPE here

Reproducer:

DynFields.builder()
    .impl(SomeClass.class, "noSuchField")
    .defaultAlwaysNull()
    .build(new Object());

// java.lang.NullPointerException: Cannot invoke "java.lang.reflect.Field.getDeclaringClass()"
//     because "this.field" is null
//   at org.apache.kyuubi.util.reflect.DynFields$UnboundField.bind(DynFields.java:89)

The sibling class in the same package already avoids this: DynMethods.UnboundMethod.NOOP overrides bind to return new BoundMethod(this, receiver), and UnboundMethod.bind carries no sentinel exemption at all.

Scope, so the severity is not overstated: nothing in Kyuubi calls defaultAlwaysNull() today, so no caller can reach this path and no user can hit it. This is hardening a vendored utility rather than fixing a reported failure. kyuubi-util's DynFields is copied from iceberg-common, which has the same defect; it is tracked upstream as apache/iceberg#17044, unfixed as of iceberg 1.11.0.

Affects Version(s)

master, and every release since 1.6.0-incubating, where DynFields was introduced (#3230)

Are you willing to submit PR?

• Yes. I would be willing to submit a PR with guidance from the Kyuubi community to fix.