Prevent NIE when unknown repl strategy with TA LBP
Prevent NIE when unknown repl strategy with TA LBP

Unknown replication strategies would raise if token aware load balancing
used.
Also fixes a more general issue where TokenAware blows up with
strategies returning an empty mapping (i.e. LocalStrategy, and Unknown).

thobbs/python-driver,mobify/python-driver,markflorisson/python-driver,mambocab/python-driver,tempbottle/python-driver,kracekumar/python-driver,mobify/python-driver,HackerEarth/cassandra-python-driver,bbirand/python-driver,jregovic/python-driver,thobbs/python-driver,vipjml/python-driver,thelastpickle/python-driver,mambocab/python-driver,beobal/python-driver,yi719/python-driver,stef1927/python-driver,kishkaru/python-driver,bbirand/python-driver,kracekumar/python-driver,beobal/python-driver,yi719/python-driver,jfelectron/python-driver,mike-tr-adamson/python-driver,datastax/python-driver,kishkaru/python-driver,jregovic/python-driver,jfelectron/python-driver,coldeasy/python-driver,markflorisson/python-driver,sontek/python-driver,coldeasy/python-driver,sontek/python-driver,HackerEarth/cassandra-python-driver,thelastpickle/python-driver,vipjml/python-driver,datastax/python-driver,mike-tr-adamson/python-driver,stef1927/python-driver,tempbottle/python-driver