Tratar model (angularjs) no qual os campos precisam ser enviados, mesmo sento nulls ou [ ]. Ideal para métodos GET ou DELETE

_treat_model_form.ng.js

function _treat_model_form() {
  let model = angular.copy(vm.filter);

  for (let m in model) {
    const value = model[m];
    model[m] =
      value == null || value.length == 0
        ? ""
        : Array.isArray(value)
        ? value.join(",") // para metodos GET|DELETE
        : value;
  }

  return model;
}
// or
function _treat_model() {
  let model = angular.copy(vm.filter);
  for (let key in model) {
    model[key] = model[key] ? model[key] : "";
  }

  return model;
}

// exemplo:
// model = { id: 1, services: [], channels: null, users: [1,2,3], status: ['started, 'ended'] }
// _treat_model_form()
// { id: 1, services: '', channels: '', users: '1,2,3', status: 'started,ended' }