Client.exec returns code null when command code is 0

When a command returns 0, node-ssh's exec always returns code: null.

It seems the faulty line is this one.
Here's a PR to fix that.

Even though this line is incorrect (the nullish coalescing operator should be used instead for example), it appears that it's not the only problem: channel.on('exit' is never emitted.

A simple way to reproduce it is with this (failing) test:

sshit('exec should return correct code', async function (t, port, client) {
  await connectWithPassword(port, client)
  const result = await client.exec('echo', ['$some', 'S\\Thing', '"Yo"'], { stream: 'both' })
  t.is(result.stdout, '$some S\\Thing "Yo"')
  t.is(result.code, 0)
})

Expected
result.code === 0

Actual
result.code === null

Start in src/index.ts around line 361 and inspect how client.exec handles the channel.on('exit') event. Reproduce the issue with the failing 'exec should return correct code' test shown in the report; done means a successful echo command returns result.code === 0 and preserves the expected stdout.