Add test case where from or to is null

Add test case to check branch that deals with changes where one of the
from or to item is null.

pkg/dyff/compare_test.go

@@ -24,11 +24,18 @@ import (
 	. "github.com/onsi/ginkgo/v2"
 	. "github.com/onsi/gomega"
 
-	"github.com/gonvenience/ytbx"
-
 	"github.com/homeport/dyff/pkg/dyff"
+
+	"github.com/gonvenience/ytbx"
+	yamlv3 "gopkg.in/yaml.v3"
 )
 
+var nullNode = &yamlv3.Node{
+	Kind:  yamlv3.ScalarNode,
+	Tag:   "!!null",
+	Value: "null",
+}
+
 var _ = Describe("Core/Compare", func() {
 	Describe("Difference between YAMLs", func() {
 		Context("Given two simple YAML structures", func() {
@@ -605,6 +612,21 @@ listY: [ Yo, Yo, Yo ]
 				}))
 			})
 
+			It("should return changes where one of the items is null", func() {
+				from := yml(`foo: null`)
+				to := yml(`foo: "bar"`)
+				results, err := compare(from, to)
+				Expect(err).To(BeNil())
+				Expect(results).NotTo(BeNil())
+				Expect(results).To(HaveLen(1))
+				Expect(results[0]).To(BeSameDiffAs(singleDiff(
+					"/foo",
+					dyff.MODIFICATION,
+					nullNode,
+					"bar",
+				)))
+			})
+
 			It("should not return order changes in named entry lists in case the ignore option is enabled", func() {
 				results, err := compare(
 					yml(`list: [ {name: A}, {name: C}, {name: B}, {name: D}, {name: E} ]`),