how to unit test method using JPA Specifications

I was playing with org.springframework.data.jpa.domain.Specifications, it's just a basic search :

 public Optional<List<Article>> rechercheArticle(String code, String libelle) {
    List<Article> result = null;

    if(StringUtils.isNotEmpty(code) && StringUtils.isNotEmpty(libelle)){
        result = articleRepository.findAll(Specifications.where(ArticleSpecifications.egaliteCode(code)).and(ArticleSpecifications.egaliteLibelle(libelle)));
    }else{
        if(StringUtils.isNotEmpty(code)){
            result= articleRepository.findAll(Specifications.where(ArticleSpecifications.egaliteCode(code)));
        }else{
            result = articleRepository.findAll(Specifications.where(ArticleSpecifications.egaliteLibelle(libelle)));
        }
    }

    if(result.isEmpty()){
        return Optional.empty();
    }else{
        return Optional.of(result);
    }
}

And that's actually working fine but I'd like to write unit tests for this method and I can't figure out how to check specifications passed to my articleRepository.findAll()

Any idea ?

Thanks