I would like my dropdown list to be nullable

here is the html

<sib-form
  data-src="https://api.happy-dev.fr/contacts/"
  fields="name, howareyou, howhelp, textfreelance, textporteur, textautre, domaine, clientlocation, workwish, project, document, budget, email, phone"
  range-howareyou="https://api.happy-dev.fr/howareyous/"
  range-howhelp="https://api.happy-dev.fr/howhelps/"
  widget-textfreelance="sib-display-freelance"
  range-workwish="https://api.happy-dev.fr/workwishs/"
  upload-url-document="/upload"
></sib-form>

and here the json request :

[
  {
    "@id": "./",
    "http://happy-dev.fr/owl/#budget": "",
    "http://happy-dev.fr/owl/#clientlocation": "",
    "http://happy-dev.fr/owl/#document": "",
    "http://happy-dev.fr/owl/#domaine": "",
    "http://happy-dev.fr/owl/#email": "xfghc",
    "http://happy-dev.fr/owl/#howareyou": {
      "@id": "https://api.happy-dev.fr/howareyous/2/"
    },
    "http://happy-dev.fr/owl/#howhelp": {
      "@id": "https://api.happy-dev.fr/howhelps/2/"
    },
    "http://happy-dev.fr/owl/#phone": "fdhg",
    "http://happy-dev.fr/owl/#project": "",
    "http://happy-dev.fr/owl/#textautre": "",
    "http://happy-dev.fr/owl/#textfreelance": "wfgcv",
    "http://happy-dev.fr/owl/#textporteur": "",
    "http://happy-dev.fr/owl/#workwish": "",
    "http://www.w3.org/2000/01/rdf-schema#label": "xhvn"
  }
]

The input "workwish" is a dropdown list which should be empty sometimes...