NullabilityHolder.reset() does not clear the isNull array

[thswlsqls]

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.

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

• 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