Fixes "Invalid MetaData" Exception in Mvc/Model/MetaData/Redis.php

Hello there.

The current implementation does not work properly, as most redis wrapper return `false` instead of `null` when no key is found (see e.g. https://github.com/nicolasff/phpredis#get).

Furthermore, the current keys are directory (`foo\bar\baz`) and not "redis" style (`foo:bar:baz`).