…astic#90259)

When a processor autoscaling decider was added in elastic#89645, an unwanted
change of behaviour sneaked in. In particular, if we cannot determine
required memory capacity, we previously returned `new AutoscalingDeciderResult(null)`
where as we now return an autoscaling result with no memory capacity
and whatever the result of the processor decider is.

Previously, if we returned a result with null capacity, the cluster
would remain as-is. Now, it is possible to cause unwanted scaling.

This commit fixes this by checking if the memory decider result
was undetermined and returns an empty result if so.

Also, some logging warnings have been added to pop up scenarios
that shouldn't happen like when the memory tracker is not called
by the master node or it has no memory estimate for anomaly detection
or analytics jobs.