Add null-pool

[garex]

Examples:

• https://github.com/symfony/cache/blob/master/Adapter/NullAdapter.php
• https://github.com/tedious/Stash/blob/master/src/Stash/Driver/BlackHole.php

Same as https://github.com/php-fig/log/blob/master/Psr/Log/NullLogger.php

Use case

Cache as an optional injectable dependency. When class user not provides it -- we take ready NullPool and use it.