Removes warnings of schema disagreements when there aren't any.

I was getting a lot of these warning messages in my app:

    WARNING:cassandra.cluster:Node 127.0.0.1 is reporting a schema disagreement: None

But this seems innocuous since there is no actual disagreement (and I'm running with a single C* node regardless.)
I don't see how this could ever be '''None''' based on the conditional and return on lines 2171-2.