Preserve null maps when projecting a map of structs

[syahra712]

Rationale for this change

Closes #4114.

When ArrowProjectionVisitor.map() rebuilds a map whose values are structs, it calls pa.MapArray.from_arrays(offsets, keys, values). That drops the validity bitmap of the original array, so null maps come back as empty maps ({}) after projection. This is the map counterpart of the list issue handled in #4065.

The fix keeps the validity and offsets buffers of the original map array and only swaps in the projected child array, so nulls (and slicing offsets) are preserved.

Are these changes tested?

Yes. Added test__to_requested_schema_null_map_of_structs in tests/io/test_pyarrow.py, covering a map with struct values, a map with primitive values, and a sliced batch. The test fails on main (null rows become []) and passes with the fix.

Are there any user-facing changes?

Null map columns with struct values are now read back as null instead of an empty map.