Fix casting error
Fix casting error

* wqflask/base/trait.py (retrieve_trait_info): If the
description_string or discription_display value return a None object,
evaluate to an empty string. Fixes errors related to *len(None)*.

pjotrp/genenetwork2,pjotrp/genenetwork2,pjotrp/genenetwork2,pjotrp/genenetwork2,genenetwork/genenetwork2,zsloan/genenetwork2,zsloan/genenetwork2,pjotrp/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,zsloan/genenetwork2,zsloan/genenetwork2