`segmented_gather()` currently assumes that null LIST rows also have
a `0` size (as defined by the difference of adjacent offsets.)

This might not hold, for example, for LIST columns that are members
of STRUCT columns whose parent null masks are superimposed on its
children. This would cause a non-empty list row to be marked null,
without compaction. This leads to errors in fetching elements of a
list row as seen in NVIDIA/spark-rapids/pull/3770.

This commit adds the handling of uncompacted LIST rows in
`segmented_gather()`.