Handle nil OCI authenticator with malformed registry

Fixes fluxcd#896

Signed-off-by: Adrien Fillon <[email protected]>

controllers/ocirepository_controller.go

@@ -672,7 +672,14 @@ func oidcAuth(ctx context.Context, url, provider string) (authn.Authenticator, e
 		opts.GcpAutoLogin = true
 	}
 
-	return login.NewManager().Login(ctx, u, ref, opts)
+	authenticator, err := login.NewManager().Login(ctx, u, ref, opts)
+	if err != nil {
+		return nil, err
+	} else if authenticator == nil {
+		return nil, fmt.Errorf("could not authenticate to OCI provider %s with URL %s", provider, url)
+	}
+
+	return authenticator, nil
 }
 
 // craneOptions sets the auth headers, timeout and user agent