Handling the way filters deal with empty lists

Filters assumed lists to always contain elements, the case
of empty lists was not handled correctly. Now we check for
empty lists in stored and expected values when applying the
filter.
Added unit tests for the findOne(knownObject) behaviour

The unit test for the FongoDbCollection better explains the actual problem.

The solution in ExpressionParser is now more general, but should be reviewed
whether it is maybe too general.
Removed unused import
Merge pull request #72 from zefixlluja/feature/empty_lists_match

Handling the way filters deal with empty lists