empty relation shouldn't be treated as null

[olaulau]

when retrieving a relation which return zero element, cortex gives null instead of a CortexCollection.
this is confusing, and forces us to tread those cases with custom code :

• if I remove one element from the relation, I can come to zero element, and have to convert my empty collection to null before save
• if the relation I get is null, I have to create a collection before adding an element to it
  any technical reason for this ?
  thanks.

[ikkez]

If i can recall correctly, it was simply because of convenient usage in templates, like
<F3:check if="{{ @user.files }}"> and so one. Having returned an empty collection would allow this condition to pass.