arrow

GH-44742: [Ruby] Fix a bug that empty struct list value can't be built (#44763)

### Rationale for this change

This codes add a list value but no struct value isn't added:  

```ruby

require "arrow"

schema = Arrow::Schema.new(
  [
   Arrow::Field.new("structs", Arrow::ListDataType.new(
     Arrow::StructDataType.new([
       Arrow::Field.new("foo", :int64),
       Arrow::Field.new("bar", :int64)
     ])
   ))
 ]
)

Arrow::RecordBatchBuilder.build(schema, [{structs: []}])
```

### What changes are included in this PR?

Don't add a list value.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #44742

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