NPE when creating MongoItemReader using a builder without specifying sorting

[bodnarIQ]

Bug description
When creating an instance of MongoItemReader using MongoItemReaderBuilder and no sorting is set, using the ItemReader results in NPE

Environment
Using dependency spring-boot-starter-batch, version 2.4.5

Steps to reproduce

    @Bean
    public ItemReader<Item> publicationReader(MongoOperations mongoOperations,
                                                     @Value("#{jobParameters['publicationId']}") String itemId) {
        return new MongoItemReaderBuilder<Item>()
                .name("currentItem")
                .template(mongoOperations)
                .collection("items")
                .targetType(Item.class)
                .pageSize(10)
                .query(query(where("_id").is(itemId)))
                //.sorts(new HashMap<>()) without this line it throws a NPE in MongoItemReader#convertToSort(Map<String, Sort.Direction> sorts); line 257
                .build();
    }

Expected behavior
When no sorting is provided, either default sorting should be used, or it should be validated, that sorting is set in MongoItemReaderBuilder#build() method. There is a validation when using queryString, but no one when using a Query object

Minimal Complete Reproducible example
See Steps to reproduce

[lcmarvin]

Hi @benas, I'm interested in this issue, I'd like to try to fix this.