Native UDFs: test coverage for empty batches and varied array encodings #6174

CometNativeUdfSuite and the native adapter tests cover every supported Spark type with nulls,
one- and two-argument calls, literal arguments in each position, and each operator a UDF can be
placed in. What they do not cover yet:

• Empty batches. A zero-row batch reaches invoke_with_args with number_rows = 0. Nothing
  checks that the kernel sees zero-length arrays and that a zero-length result passes the row count
  check.
• Sliced arrays. An array with a non-zero offset (as produced by a LIMIT or a filter over a
  batch) crosses the C Data Interface with its offset, and the kernel has to honor it. Nothing
  exercises a non-zero offset in either direction.
• Dictionary-encoded input. A dictionary-encoded string column from a Parquet scan reaches
  the UDF as whatever Comet's scan hands the projection. The type the kernel sees then depends on
  whether the dictionary was unpacked first, and return_field implementations that match on
  DataType::Utf8 would reject it.

Follow-up to #4459.