Fix NPE in /_cat/nodes

Query Cache and Suggest stats were introduced in v1.4.0_beta1 and 1.2.0 respectively, therefore they can be null if stats are received from older nodes in the cluster.

Fixes #6297

This problem can occur only 1.x since current master isn't backward compatible with previous versions of elasticsearch anyway. So, I am opening this PR against 1.x not master. Not sure if this is right thing to do.