7.5+ for SLM requires [stats] object to exist in the cluster state.
When doing an in-place upgrade from 7.4 to 7.5+ [stats] does not exist
in cluster state, result in an exception on startup [1].
This commit moves the [stats] to be an optional object in the parser
and if not found will default to an empty stats object.
[1] Caused by: java.lang.IllegalArgumentException: Required [stats]
cf175da5a9