arrow

GH-44918: [Ruby] Fix a bug that empty list is ignored in builder (#44933)

### Rationale for this change

`Arrow::ListArrayBuilder#append_value` with `[]` must append an empty list as an element. In general, it works but it doesn't work when list item is struct or list. Because `Arrow::{List,Struct}ArrayBuilder#append` without arguments appends an element. It's for a backward compatibility. But it has a problem for this case.

For example, the following case has this problem:

```ruby
item_type = Arrow::StructDataType.new([{name: "visible", type: :boolean}])
data_type = Arrow::ListDataType.new(name: "struct", data_type: item_type)
builder = Arrow::ListArrayBuilder.new(data_type)
builder.append_value([])
array = builder.finish
array.to_a # => must be [[]] but [] without this change
```

### What changes are included in this PR?

This should be fixed by GH-44763 but the fix was wrong. This change fixes wrong `return if` location.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #44918

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>