Allow annotation without value in Rasa DIET

[NoB0]

Some of the training examples for Rasa DIET annotator assign a value to an entity when it is not needed. For example:Do you have any specific [genres](GENRE) in mind?. In this case, the value of slot should be None instead of genres.

Investigate if the training examples can be modified to remove the value. If not, apply post-processing in the method get_annotations of the annotator.