fix: prevent SIGABRT when adding nullable column to multi-segment collection (#415) (#416)

When add_column is called on a multi-segment collection with a nullable
field and no expression, segment.cc previously sliced an Arrow ChunkedArray
with an offset that exceeded the array length, triggering SIGABRT in
Arrow's chunked_array.cc:170 assertion.

Fix the slicing logic in segment.cc to materialize null values per segment.
Add comprehensive tests in collection_test.cc and segment_test.cc covering
multi-segment add_column scenarios (nullable/non-nullable, with/without
expression, with/without unflushed data, drop+re-add).