fix: check for token before revoking (actions#30)

Check before trying to revoke the token, in case the token generation
failed. Otherwise the post step will throw an error.

---------

Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>

lib/post.js

@@ -10,6 +10,8 @@ import { request } from "@octokit/request";
 export async function post(core, request) {
   const token = core.getState("token");
 
+  if (!token) return;
+
   await request("DELETE /installation/token", {
     headers: {
       authorization: `token ${token}`,