scheduler: fix panic in system jobs when nodes filtered by class

In the system scheduler, if a subset of clients are filtered by class,
we hit a code path where the `AllocMetric` has been copied, but the
`Copy` method does not instantiate the various maps. This leads to an
assignment to a nil map. This changeset ensures that the maps are
non-nil before continuing.

The `Copy` method relies on functions in the `helper` package that all
return nil slices or maps when passed zero-length inputs. This
changeset to fix the panic bug intentionally defers updating those
functions because it'll have potential impact on memory usage. See
#11564 for more details.

scheduler/scheduler_system.go

@@ -287,9 +287,16 @@ func mergeNodeFiltered(acc, curr *structs.AllocMetric) *structs.AllocMetric {
 
 	acc.NodesEvaluated += curr.NodesEvaluated
 	acc.NodesFiltered += curr.NodesFiltered
+
+	if acc.ClassFiltered == nil {
+		acc.ClassFiltered = make(map[string]int)
+	}
 	for k, v := range curr.ClassFiltered {
 		acc.ClassFiltered[k] += v
 	}
+	if acc.ConstraintFiltered == nil {
+		acc.ConstraintFiltered = make(map[string]int)
+	}
 	for k, v := range curr.ConstraintFiltered {
 		acc.ConstraintFiltered[k] += v
 	}