handle Optional Null Iterator selector

capability/policy/selector/selector.go

@@ -93,9 +93,20 @@ func resolve(sel Selector, subject ipld.Node, at []string) (ipld.Node, []ipld.No
 		// 2nd level: handle different node kinds (list, map, string, bytes)
 		switch {
 		case seg.Iterator():
-			if cur == nil {
+			if cur == nil || cur.Kind() == datamodel.Kind_Null {
 				if seg.Optional() {
-					cur = nil
+					// build empty list
+					nb := basicnode.Prototype.List.NewBuilder()
+					assembler, err := nb.BeginList(0)
+					if err != nil {
+						return nil, nil, err
+					}
+
+					if err = assembler.Finish(); err != nil {
+						return nil, nil, err
+					}
+
+					return nb.Build(), nil, nil
 				} else {
 					return nil, nil, newResolutionError(fmt.Sprintf("can not iterate over kind: %s", kindString(cur)), at)
 				}

capability/policy/selector/supported_test.go

@@ -30,7 +30,7 @@ func TestSupportedForms(t *testing.T) {
 	for _, testcase := range []Testcase{
 		{Name: "Identity", Selector: `.`, Input: `{"x":1}`, Output: `{"x":1}`},
 		{Name: "Iterator", Selector: `.[]`, Input: `[1, 2]`, Output: `[1, 2]`},
-		{Name: "Optional Null Iterator", Selector: `.[]?`, Input: `null`, Output: `()`},
+		{Name: "Optional Null Iterator", Selector: `.[]?`, Input: `null`, Output: `[]`},
 		{Name: "Optional Iterator", Selector: `.[][]?`, Input: `[[1], 2, [3]]`, Output: `[1, 3]`},
 		{Name: "Object Key", Selector: `.x`, Input: `{"x": 1 }`, Output: `1`},
 		{Name: "Quoted Key", Selector: `.["x"]`, Input: `{"x": 1}`, Output: `1`},