Unable to use assign providing an empty list of children objects

[emanoelxavier]

The method assign in the RestObject should allow for providing a list of empty children. According to the REST Api documentation this is the mechanism that should be used to unassign all the children from a parent. Current this method is enforcing that the list of children cannot be empty which is inconsistent with the REST Api behavior.

[emanoelxavier]

@rvichery please take a look at this issue as it is blocking integration with OSC when we try to clean up objects. I am sending a PR to fix the problem.

[martin-leclerc]

Code review done on #5.

I will approve PR once the 3 change requests have been sumbitted.