[C++] StructBuilder::AppendNull() Should Use AppendNull() for Children Instead of AppendEmpty()

### StructBuilder::AppendNull() Should Use AppendNull() for Children Instead of AppendEmpty()

1. The current implementation of StructBuilder appends empty to each child vector for each appended null.
2. In some scenarios, I try to extract child[0], and theoretically, the row that was appended as null should also be null. However, this is not the case in practice (because the current implementation uses AppendEmpty for child arrays).
I would like to rewrite the code like this:

Status AppendNull() final {
for (const auto& field : children_) {
ARROW_RETURN_NOT_OK(field->AppendNull());
}
return Append(false);
}

### Component(s)

C++