dagcbor: test that undefined token parses and becomes treated as null.

(Made a one-off test for this.  It should also be a fixture in
the ipld/ipld metarepo, but we don't already have those wired up,
so I'm making the more incremental diff today.)

codec/dagcbor/unmarshal_test.go

@@ -4,6 +4,8 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/ipld/go-ipld-prime/datamodel"
+
 	qt "github.com/frankban/quicktest"
 
 	"github.com/ipld/go-ipld-prime/node/basicnode"
@@ -38,4 +40,13 @@ func TestFunBlocks(t *testing.T) {
 		err := Decode(nb, buf)
 		qt.Assert(t, err, qt.Equals, ErrAllocationBudgetExceeded)
 	})
+	t.Run("undef", func(t *testing.T) {
+		// This fixture tests that we tolerate cbor's "undefined" token (even though it's noncanonical and you shouldn't use it),
+		// and that it becomes a null in the data model level.
+		buf := strings.NewReader("\xf7")
+		nb := basicnode.Prototype.Any.NewBuilder()
+		err := Decode(nb, buf)
+		qt.Assert(t, err, qt.IsNil)
+		qt.Assert(t, nb.Build().Kind(), qt.Equals, datamodel.Kind_Null)
+	})
 }