[ExpiringCache] Fix the get() method on expired entries, some tweaks.

According to the CacheInterface, `get()` should return either the cached value or null.

This is what I have implemented in the `ExpiringCache` (previously get would return the value if even if the entry is outdated).

It is ok ? I have noticed that the `FileSystemCache` and `ConfigCache` throw and an exception which is yet an other behavior.