Combine apple juice and quinoa in a saucepan. Bring to a boil; reduce heat and simmer, partially covered, until quinoa is tender, about 20 minutes. Drain excess water and transfer to a large bowl. Whisk agave nectar, safflower oil, poppy seeds, and mustard together in a small bowl to make dressing. Chill quinoa and dressing in the refrigerator, at least 4 hours. Fold apple, cranberries, cashews, onion, and salt into chilled quinoa. Add dressing; mix well to combine.