message: refactor: storage returns undefined
  instead of null for unknown keys
diff: @@ -165,7 +165,7 @@ export default class Storage implements IStorage {
  public getItem(type: string, key: string): any;
  public getItem(key: string): any;
  public getItem(): any {
  - let key;
  + let key: string;
  if (arguments.length === 1) {
  key = arguments[0];
  @@ -175,6 +175,10 @@ export default class Storage implements IStorage {
  key = this.getPrefix() + key;
  + if (!Storage.backend.hasOwnProperty(key)) {
  + return undefined;
  + }
  +
  let value = Storage.backend.getItem(key);
  return this.parseValue(value);
  