Can cache empty collections

I should be able to cache an "empty" collection.

I have a some objects, where 90% of these have on-to-many relations with zero associated elements.
This causes doctrine to run a query each time, instead of cache it as empty relation.
Can cache many to many empty relations
Test empty collections second level cache
Merge remote-tracking branch 'origin/master' into patch-1
Typo fix on many-to-many assocaition cascade test
Merge pull request #950 from goetas/patch-1

[SLC] Cache empty collections
Update doc with latest news about extra lazy assoc
Update extra-lazy-associations.rst
Merge pull request #953 from goetas/patch-1

Update doc with latest news about extra lazy assoc