NullabilityHolder.reset() does not clear the isNull array

### Apache Iceberg version

None

### Query engine

None

### Please describe the bug 🐞

`NullabilityHolder.reset()` resets `numNulls` but not the `isNull` byte array, leaving stale null markers from the previous batch.

```java
NullabilityHolder holder = new NullabilityHolder(10);
holder.setNull(3);
holder.reset();
holder.hasNulls(); // false
holder.isNullAt(3); // 1 — stale
```

The other two arrays (`nulls`, `nonNulls`) are immutable `arraycopy` source buffers and do not need clearing.

### Willingness to contribute

- [x] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Locate the Java implementation of NullabilityHolder and inspect reset(), setNull(), hasNulls(), and isNullAt(). Reproduce the reported sequence with a focused test, then verify that reset removes the stale marker while preserving the existing behavior of the other arrays.

Written by the indexing model from the issue text.