IPC writer should truncate string array with all empty string

[JasonLi-cn]

Describe the bug

For an Array like StringArray that contains two Buffers, if its Value Buffer is empty and its Value Offset Buffer is not empty, which means that it contains all elements of the empty string. In this case, let slice = array.slice(0, n); sending slice will result in a truncation failure because buffer_need_truncate() returns false.

To Reproduce

    #[test]
    fn truncate_ipc_string_array_with_all_empty_string() {
        fn create_batch() -> RecordBatch {
            let schema = Schema::new(vec![
                Field::new("a", DataType::Utf8, true),
            ]);
            let a = StringArray::from(vec![Some(""), Some(""), Some(""), Some(""), Some("")]);
            RecordBatch::try_new(Arc::new(schema), vec![Arc::new(a)])
                .unwrap()
        }

        let record_batch = create_batch();
        let record_batch_slice = record_batch.slice(0, 1);
        let deserialized_batch = deserialize(serialize(&record_batch_slice));

        // actual
        assert_eq!(serialize(&record_batch).len(), serialize(&record_batch_slice).len());
        
        // expected
        // assert!(serialize(&record_batch).len() > serialize(&record_batch_slice).len());

        assert_eq!(record_batch_slice, deserialized_batch);
    }

Expected behavior

        // expected
        assert!(serialize(&record_batch).len() > serialize(&record_batch_slice).len());

Additional context