feat: support NullType in row-to-Arrow conversion and shuffle

[mbutrovich]

Which issue does this PR close?

Closes #4457.

Rationale for this change

NullType columns currently break Comet at the row-to-Arrow boundary. Utils.toArrowType throws UnsupportedOperationException for NullType, which surfaces in two places:

1. CometLocalTableScanExec when a LocalTableScan contains a NullType column (the case in Queries with NullType aggregate fails when native LocalTableScanExec is enabled #4457, e.g. SELECT max(col) FROM VALUES (NULL), (NULL) AS t(col)).
2. CometShuffleExchangeExec when a Spark LocalTableScanExec with a NullType column feeds a Comet shuffle.

NullType is well-defined in Arrow (ArrowType.Null) and the Spark ArrowWriter already has a NullWriter case, so the right fix is to support it end-to-end rather than fall back. This PR is an alternative to #4458, which adds a LocalTableScanExec-only fallback and leaves the shuffle path broken.

What changes are included in this PR?

• Utils.toArrowType maps NullType to ArrowType.Null.
• CometShuffleExchangeExec.supportedSerializableDataType accepts NullType for both native and columnar shuffle.
• Native shuffle row reader (native/shuffle/src/spark_unsafe/row.rs) handles NullType.

How are these changes tested?

New regression tests:

• CometExecSuite "CometLocalTableScanExec handles NullType nested in struct/array/map" covers NullType nested under StructType, ArrayType, and MapType through CometLocalTableScanExec.
• CometColumnarShuffleSuite "columnar shuffle with NullType passthrough column" covers JVM-input columnar shuffle with a NullType column. Replaces the older "Fallback to Spark for unsupported input besides ordering" test, which asserted the previous fallback behavior.
• CometNativeShuffleSuite "native shuffle with NullType passthrough column" covers native shuffle with a Comet LocalTableScan source containing a NullType column. Gated on spark.comet.exec.localTableScan.enabled=true because native shuffle requires Comet input.