fix: GetKeychains always returns at least 1 keychain

GetDescriptor iterate over keychains returned by GetKeychains, so if
GetKeychains returns 0 keychain, GetDescriptor will return a nil
descriptor

Author: plaffitt

internal/registry/keychain.go

@@ -50,6 +50,8 @@ func GetKeychains(repositoryName string, pullSecrets []corev1.Secret) ([]authn.K
 		})
 	}
 
+	keychains = append(keychains, authn.DefaultKeychain)
+
 	return keychains, nil
 }