Ensure nullsmax for descending ordered pools

Closes #4709

compiler/kernel/op.go

@@ -600,8 +600,7 @@ func (b *Builder) compile(o dag.Op, parents []zbuf.Puller) ([]zbuf.Puller, error
 		if err != nil {
 			return nil, err
 		}
-		nullsMax := o.Order == order.Asc
-		cmp := expr.NewComparator(nullsMax, !nullsMax, e).WithMissingAsNull()
+		cmp := expr.NewComparator(true, o.Order == order.Desc, e).WithMissingAsNull()
 		return []zbuf.Puller{merge.New(b.octx, parents, cmp.Compare)}, nil
 	case *dag.Combine:
 		return []zbuf.Puller{combine.New(b.octx, parents)}, nil

runtime/op/meta/lister.go

@@ -104,7 +104,7 @@ func initObjectScan(snap commits.View, sortKey order.SortKey) []*data.Object {
 }
 
 func sortObjects(objects []*data.Object, o order.Which) {
-	cmp := expr.NewValueCompareFn(o, o == order.Asc) //XXX is nullsMax correct here?
+	cmp := expr.NewValueCompareFn(o, true)
 	lessFunc := func(a, b *data.Object) bool {
 		aFrom, aTo, bFrom, bTo := &a.Min, &a.Max, &b.Min, &b.Max
 		if o == order.Desc {

runtime/op/meta/ztests/nulls-max.yaml

@@ -0,0 +1,29 @@
+# This test ensures that nulls are max for either descending or ascending 
+# ordered pools
+
+script: |
+  export ZED_LAKE=test
+  zed init -q
+  for o in asc desc; do
+    echo "// order $o"
+    zed create -q -use -orderby ts:$o $o
+    echo '{ts:150} {ts:null}' | zed load -q -
+    echo '{ts:1}' | zed load -q -
+    zed query -z "from $o:objects | drop id, size"
+    echo "// ==="
+    zed query -z 'head 1'
+  done
+
+outputs:
+  - name: "stdout"
+    data: |
+      // order asc
+      {min:1,max:1,count:1(uint64)}
+      {min:150,max:null,count:2(uint64)}
+      // ===
+      {ts:1}
+      // order desc
+      {min:150,max:null,count:2(uint64)}
+      {min:1,max:1,count:1(uint64)}
+      // ===
+      {ts:null}