Fix Storage interface

- Method getItem returns string.
- It's possible to use property accessors to handle values in storage but this is not
   the part of standard. Also there can be confusion because any object can be written
   into property of storage object but when property is read string value is always
   returned.

https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Storage
Introduce null-friendly Storage wrapper

Null-returning methods are replaced with option-retuning ones.