Add assertion for get autoscaling decision API test

This commit adds a match assertion to the get autoscaling decision REST
test.

Author: jasontedor

x-pack/plugin/autoscaling/qa/rest/src/test/resources/rest-api-spec/test/autoscaling/get_autoscaling_decision.yml

@@ -2,3 +2,5 @@
 "Test get autoscaling decision":
   - do:
       autoscaling.get_autoscaling_decision: {}
+
+  - match: { "decisions": [] }