explicitly return a string for String method instead of panic

Signed-off-by: Adam D. Cornett <adc@redhat.com>

Author: acornett21

pkg/imagename/imagename.go

@@ -19,6 +19,8 @@ const (
 	ExplicitTag
 	// ExplicitNamespace forces a namespace in the repo, will use "library" if no namespace
 	ExplicitNamespace
+	// invalidImageNameString used in Stringer method to represent invalid ImageName
+	invalidImageNameString = "<invalid>"
 )
 
 var (
@@ -121,10 +123,14 @@ func (imageName *ImageName) Enclose(organization string) {
 // String returns the string representation of the image using
 // the registry and tag formatting.
 func (imageName *ImageName) String() string {
-	result, err := imageName.ToString(Registry | Tag)
+	// Guard against nil receiver; Stringer implementations should not panic.
+	if imageName == nil {
+		return invalidImageNameString
+	}
+	result, err := imageName.ToString(DefaultGetStringOptions)
 
 	if err != nil {
-		panic(err)
+		return invalidImageNameString
 	}
 
 	return result

pkg/imagename/imagename_test.go

@@ -59,6 +59,19 @@ var _ = Describe("ImageNameParse", func() {
 		),
 	)
 
+	DescribeTable("String() returns <invalid> for invalid ImageNames",
+		func(imageName *ImageName, expectedString string) {
+			// String() should never panic, even for invalid ImageNames
+			Expect(imageName.String()).To(Equal(expectedString))
+		},
+		Entry("nil ImageName", (*ImageName)(nil), invalidImageNameString),
+		Entry("empty ImageName", &ImageName{}, invalidImageNameString),
+		Entry("missing repo", &ImageName{Registry: "example.com", Namespace: "namespace", Repo: "", Tag: "latest"}, invalidImageNameString),
+		Entry("only registry", &ImageName{Registry: "example.com"}, invalidImageNameString),
+		Entry("only namespace", &ImageName{Namespace: "namespace"}, invalidImageNameString),
+		Entry("only tag", &ImageName{Tag: "latest"}, invalidImageNameString),
+	)
+
 	DescribeTable("encloses",
 		func(repo, organization, enclosedRepo, registry, tag string) {
 			reference := repo