cmd/gitops-pusher: fix precedence when id token env var is empty

Fix precedence logic to skip federated identity logic when the
associated environment variables are empty.

Updates https://github.com/tailscale/gitops-acl-action/issues/71

Signed-off-by: Mario Minardi <mario@tailscale.com>

cmd/gitops-pusher/gitops-pusher.go

@@ -252,7 +252,7 @@ func getCredentials() (*http.Client, string) {
 				TokenURL:     fmt.Sprintf("https://%s/api/v2/oauth/token", *apiServer),
 			}
 			client = oauthConfig.Client(context.Background())
-		} else if idok {
+		} else if idok && idToken != "" && oiok && oauthId != "" {
 			if exchangeJWTForToken, ok := tailscale.HookExchangeJWTForTokenViaWIF.GetOk(); ok {
 				var err error
 				apiKeyEnv, err = exchangeJWTForToken(context.Background(), fmt.Sprintf("https://%s", *apiServer), oauthId, idToken)