Enhancing Rasa: No Utterance if no Entity

• Bot returns 'Birthdate of None is None' if no physicist entity is recognized.

# question
> what did Max Mustermann research
# bot-utterance
> Birthdate of None is None

• Instead, there should be a default utterance like

# question
> what did Max Mustermann research
# bot-utterance
> Sorry, I do not recognize Max Mustermann as a famous physicist. :(