Function take_sparse_union_child 

fn take_sparse_union_child<IndexType: ArrowPrimitiveType, const CHECKED: bool>(
    values: &dyn Array,
    represent_nulls: bool,
    indices: &PrimitiveArray<IndexType>,
) -> Result<ArrayRef, ArrowError>

Takes a sparse union child for indices.

Null take indices select one child that can represent a null (take_union_type_ids). Values in the other children at those positions are unspecified, so they are taken with dummy indices instead of introducing nulls that would contradict field metadata.