Adds missing magic method __nonzero__

This is called each time this object is an argument to a
conditional statement. If this method is not present,
__getattr__ would be called instead. That would cause an
endless recursive call. Strangely, this was not an issue
before.

Error encountered running:
conn = wmi.WMI(moniker='//./root/MSCluster')
conn.MSCluster_Cluster

Also, this should improve performance as well,
since it avoids useless recursive calls.