DateField shouldn't be mandatory

If I have a DateField on one of my modele, then I cannot do without passing a value to that field on resource creation / edition. If I avoid passing the date field like :

{
  "@id": "./",
  "http://happy-dev.fr/owl/#address": "Gare de Lyon Galerie Diderot",
  "http://happy-dev.fr/owl/#city": "Paris",
  "http://happy-dev.fr/owl/#crowdfunding": "",
  "http://happy-dev.fr/owl/#date": "",
  "http://happy-dev.fr/owl/#description": "Les grenouilles zomafites sont une espèce en voie de disparition",
  "http://happy-dev.fr/owl/#end_time": "",
  "http://happy-dev.fr/owl/#image": "Manifestation",
  "http://happy-dev.fr/owl/#location": [
    "",
    "Gare de Lyon Galerie Diderot, Paris 75012, France"
  ],
  "http://happy-dev.fr/owl/#num_of_members": "",
  "http://happy-dev.fr/owl/#postcode": 75012,
  "http://happy-dev.fr/owl/#start_time": "",
  "http://happy-dev.fr/owl/#submit-btn": "Je créé une nouvelle action",
  "http://happy-dev.fr/owl/#theme": {
    "@id": "http://127.0.0.1:8000/api/actionthemes/1/"
  },
  "http://www.w3.org/2000/01/rdf-schema#label": "Protégeons la biodiversité",
  "http://www.w3.org/2003/01/geo/wgs84_pos#lat": 48.8448057,
  "http://www.w3.org/2003/01/geo/wgs84_pos#long": 2.3734794
}

... I get the following 400 :

{
    "date": [
      "Date has wrong format. Use one of these formats instead: YYYY-MM-DD."
    ],
    "@context": "https://cdn.happy-dev.fr/owl/hdcontext.jsonld"
  }

... even though I defined the filed as "blankable" and nullable :

    date = models.DateField(blank=True, null=True, verbose_name='Date')