Fix NPE in MlMemoryAutoscalingDecider

Author: jan-elastic

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/autoscaling/MlMemoryAutoscalingDecider.java

@@ -259,7 +259,7 @@ public MlMemoryAutoscalingCapacity scale(
             .map(result -> {
                 MlMemoryAutoscalingCapacity capacity = ensureScaleDown(
                     result,
-                    MlMemoryAutoscalingCapacity.from(context.currentCapacity()).build()
+                    context.currentCapacity() == null ? null : MlMemoryAutoscalingCapacity.from(context.currentCapacity()).build()
                 );
                 if (capacity == null) {
                     return null;