Update lesk.py

A crude fix for a bug that was coming up when a very obscure or arguably unseen word was being used in simple_lesk(). E.g. using the word 'overfitting' was leading to an index overlow error. 

As a quickfix, I've added a check right before the return to check the size of the list 'ranked_synsets' and to force a Nonetype return when it is empty.