json='[ { "Data": 2019, "IdJudet": "09,18" }, { "Data": 2019, "IdJudet": "09,18, 20, 03" }, { "Data": 2019, "IdJudet": "09" }, { "Data": 2019, "IdJudet": null } ]';

obj = JSON.parse(json)
const ids = obj
   // get only the required field
  .map((item) => item.IdJudet)
   // filter away null values and empty strings
  .filter(Boolean)
   // split by comma and trim spaces
  .map((IdJudet) => IdJudet.split(",").map(id => id.trim()))
   // reduce arrays to your result
  .reduce((result, ids) => result.concat(ids), [])

// remove duplicates
const result = ids.filter((id, i) => ids.indexOf(id) === i)

console.log(result);