PYTHON-181 - Make TokenAware tolerant of queries with no keyspace set

It looks like this is probably safe in this case, but in general, it's not a good idea to use mutable class-level attributes that are intended to be changed per-instance.  This is because all instances of this class will share the same mutable data structure.  Instead, either use an immutable value (like an empty tuple) or leave it as ``None`` and set it in the constructor.