ScanIterator.sliceArray crashes on NullArray (all-null dtype columns)

[dfa1]

Root cause

ScanIterator.sliceArray (line ~776) has no case for NullArray, so it throws:

VortexException: scan: cannot slice shared array of type NullArray

This happens when a column has dtype DType.Null (all-null, no values, no validity bitmap). Such columns are decoded as NullArray by NullEncodingDecoder. When the scan iterator needs to slice the decoded chunk to a row window, it falls through to the default error arm.

Affected slug

nyc-property-sales — column easement has dtype null, written by vortex-data 0.69.0 as a vortex.null encoding, decoded as NullArray. Export crashes immediately on the first chunk.

Fix

Add a NullArray case to ScanIterator.sliceArray that returns a new NullArray of the requested row count. A NullArray carries no data, so slicing is just a row-count rebind.

case NullArray ignored -> new NullArray(rows);