message: prefer readable checks vs array index thing
diff: import ErrorBag from './errorBag';
  import Dictionary from './dictionary';
  import { messages } from '../locale/en';
  -import { isObject, isCallable, toArray, warn, createError, assign, find } from './utils';
  +import { isObject, isCallable, toArray, warn, createError, assign, find, isNullOrUndefined } from './utils';
  import Field from './field';
  import FieldBag from './fieldBag';
  @@ -766,7 +766,7 @@ export default class Validator {
  * @param {Boolean} silent
  */
  _validate (field, value, silent = false) {
  - if (!field.isRequired && ~[null, undefined, ''].indexOf(value)) {
  + if (!field.isRequired && (isNullOrUndefined(value) || value === '')) {
  return Promise.resolve(true);
  }
  