[BUG] user utterance from user simulator is empty randomly about convlab-2 HOT 4 CLOSED

user in []
user out [['Inform', 'Restaurant', 'none', 'none'], ['Inform', 'Restaurant', 'Price', 'expensive'], ['Inform', 'Restaurant', 'Area', 'centre'], ['Inform', 'Restaurant', 'Food', 'portuguese']]
user: I need a restaurant . It just needs to be expensive . I am also in the market for a new restaurant . Is there something in the centre ? Do you have portuguese food ?
sys: I did not find any portuguese restaurants . Give me a moment and I will search for unusual ones . We need some more information . Where are looking to eat ?
user in [['Request', 'Restaurant', 'Area', '?'], ['NoOffer', 'Restaurant', 'none', 'none'], ['NoOffer', 'Restaurant', 'Food', 'portuguese']]
user out [['Inform', 'Restaurant', 'Area', 'centre'], ['Inform', 'Restaurant', 'Price', 'cheap']]
user: I ' m so hungry - can you find me a place to eat in the centre ? I 'd like a cheap priced one .
sys: It is in the centre area . Would you like to try nandos city centre ? They are in the cheap price range . I will book it for you and get a reference number ?
user in [['Inform', 'Booking', 'none', 'none'], ['Recommend', 'Restaurant', 'Area', 'centre'], ['Recommend', 'Restaurant', 'Name', 'nandos city centre'], ['Recommend', 'Restaurant', 'Price', 'cheap']]
user out [['Inform', 'Restaurant', 'NotBook', 'none']]
user: 
sys: 
user in []
user out [['Request', 'Restaurant', 'Post', '?']]
user: Can I get the postcode for the restaurant ?
sys: How does the nandos city centre sound ? Their postcode is cb23ar. Is there anything else I can help you with today ?
user in [['reqmore', 'general', 'none', 'none'], ['Recommend', 'Restaurant', 'Name', 'nandos city centre'], ['Recommend', 'Restaurant', 'Post', 'cb23ar']]
user out [['bye', 'general', 'none', 'none']]
user: Ok , have a good day . Goodbye .
sys: Did you need to book the nandos city centre ?