Add a default value for drop_down_list_value to avoid validation error on new type_de_champ

app/javascript/components/TypesDeChampEditor/index.js

@@ -22,6 +22,7 @@ class TypesDeChampEditor extends Component {
       type_champ: 'text',
       types_de_champ: [],
       private: props.isAnnotation,
+      drop_down_list_value: '--Premier élément du menu--\n',
       libelle: `${
         props.isAnnotation ? 'Nouvelle annotation' : 'Nouveau champ'
       } ${props.typeDeChampsTypes[0][0]}`