Changed the default return value for methods that return a collection interface to return an instance of an empty collection

[GustavoRusso]

This change tries to reduce the number of methods stubs needed to stub an object.

The reason why we use a stub is to communicate that the object is needed but does not act for the purpose that we want to test.

Currently, if there is a call to a method of a stub that returns a collection, the stub object will return a null that will probably cause a null reference exception.
We will have to add a stub espectation for the method even if it does not act on what we want to test.

It is usual that the real implementation of a method that return a collection, return a empty collection instead of a null.

Therefore, if the stub object returns an empty collection instead of a null, will be more similar to the real behavior of the real class, and in addition, it will prevent the need of additional stub calls.

[alaendle]

Even if it's a breaking change - i'll guess it would be useful to return stubs for this interfaces. +1