Allow behaviors to alter the definition of "is empty"

Issue Summary

I'm working on a module providing an hierarchical structure implemented as an Entity Reference behavior: http://drupal.org/sandbox/damz/1427028

In the case of root nodes, I want the "-None-" value to be still stored in the tree, because it is significant. In that case, I need to be able to tell Entity Reference "it's ok to store empty references", thus overriding entityreference_field_is_empty().