bug: Promise success clears when 2nd promise has undefined success

**What is the current behavior?**

When calling two promises in a row, if one has a `success` defined, but the other does not, then the first one closes instantly. Example:

```
await toast.promise(async () => {}, {
success: 'Success on 1'
})
await toast.promise(async () => {}, {
// success: 'Success on 2',
error: 'Error on 2'
})
```

If you uncomment the `success` on the 2nd one, then all works well.

---

**What is the expected behavior?**

`success` shouldn't be required. When absent, it shouldn't affect other toasts.

Note: Adding a `toastId` does not help.

---

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

React v18.2.0; not tested in other versions
Tested in Chrome and Firefox

BTW, awesome package! Simple and beautiful.