tests(e2e): Null image version check improvements (#830)

e2e/cmd/build_base_image/00_check_vm_image/main.go

@@ -109,7 +109,7 @@ func action(ctx context.Context, _ *command.Command) error {
 	// - Z: patch version, incremented for consecutive builds of the same minor version, starts at 0
 
 	// Handle case where we have no custom image at all
-	if latestCustomImageVersion == "0.0.0" || force {
+	if latestCustomImageVersion == az.NullImageVersion || force {
 		fmt.Println(latest.URN)
 		return nil
 	}

e2e/cmd/build_base_image/02_create_vm_template/main.go

@@ -82,7 +82,7 @@ func action(ctx context.Context, cmd *command.Command) error {
 	}()
 
 	// If the version is empty, we need to create the image definition
-	if latestImageVersion == "" {
+	if latestImageVersion == az.NullImageVersion {
 		log.Infof("Creating image definition %q", imageDefinition)
 		_, _, err := az.RunCommand(ctx, "sig", "image-definition", "create",
 			"--resource-group", "AD",

e2e/internal/az/image.go

@@ -11,6 +11,9 @@ import (
 	"golang.org/x/exp/slices"
 )
 
+// NullImageVersion is the version returned when no image version is found.
+const NullImageVersion = "0.0.0"
+
 // Image contains information about an Azure image.
 type Image struct {
 	Architecture string `json:"architecture"`
@@ -104,7 +107,7 @@ func (i Image) isGen2Image() bool {
 // LatestImageVersion returns the latest image version for the given image definition.
 // If no version exists, "0.0.0" is returned.
 func LatestImageVersion(ctx context.Context, imageDefinition string) (string, error) {
-	latestVersion := "0.0.0"
+	latestVersion := NullImageVersion
 
 	out, _, err := RunCommand(ctx, "sig", "image-version", "list",
 		"--resource-group", "AD",