Fix empty keychain panic (#110)

Signed-off-by: m.nabokikh <[email protected]>

pkg/registry/checker.go

@@ -326,7 +326,11 @@ func check(ref name.Reference, kc authn.Keychain, registryTransport *http.Transp
 	// Fallback to default keychain if image is not found in the provided one.
 	// This is a behavior that is close to what CRI does. Because, there is maybe an image pull secret, but with
 	// the wrong credentials. Yet, the image may be available with the default keychain.
-	kc = authn.NewMultiKeychain(kc, authn.DefaultKeychain)
+	if kc != nil {
+		kc = authn.NewMultiKeychain(kc, authn.DefaultKeychain)
+	} else {
+		kc = authn.DefaultKeychain
+	}
 
 	_, imgErr = remote.Head(
 		ref,