Fix exception when all options are passed via environment variables

[kezhenxu94]

When starting the egg application with plain node command and all arguments are passed via environment variables, exception is thrown,

egg.startCluster(
  {
    workers,
    baseDir: __dirname,
    port: Number(process.env.PORT) || 3000,
  },
  () => {
    console.info('start server');
  },
);

command: node --require skyapm-egg-require server.js

exception:

undefined:1