prevent table value display when value is null

Hi, 

I noticed a couple of logs such as this one, where nil value show up for "table": 

ef169132 [Cassandra Java Driver worker-1] 2013-06-21 11:46:50,250 DEBUG com.datastax.driver.core.ControlConnection  - [Control connection] Refreshing schema for stats.null

The format of the logging message is now the same as the one found in Cluster.java for the same event.