avoid undefined method `foo' for nil:NilClass when state is nil

Sometimes node state can be nil, e.g. (AFAICS) if logrotate restarts
chef daemons at the same time as a Crowbar API query:

  https://bugzilla.novell.com/show_bug.cgi?id=838142#c3

We should consistently return a String ("unknown") even in these cases,
to ensure that code like

  node.state.titlecase

doesn't barf.