CmkKpiStatCard: render a no-data state

value becomes optional; omitting it renders an em dash in place of
the number (unit stays visible), and drops the curve, delta, and
state badge, and shows a short body note. Card colorization is also
suppressed - there's no state to color.

CMK-38083

Change-Id: Id5aef7bd469c8f0388d0ab66db64485eb594e7ca