Pass an emoty confOverlay to make sure CDH 4.3.0 hiveserver2 does not blow up

There is an issue with using rbhive with CDH 4.3.0 and hiveserver2: https://issues.cloudera.org/browse/DISTRO-500. Basically, it is impossible to use rbhive with CDH 4.3.0 hiveserver2 deployments. They have fixed it now and are going to release the fix in the next CDH release, but until then it would be nice to still be able to use rbhive.

I've made this simple change to pass an empty hash for the confOverlay parameter instead of relying the default null value being interpreted by the server correctly.