The below error is displayed during some tests

```js
ERROR: 'Unhandled Promise rejection:', 'Cannot read properties of null (reading 'markForCheck')', '; Zone:', 'ProxyZone', '; Task:', 'Promise.then', '; Value:', TypeError: Cannot read properties of null (reading 'markForCheck')
TypeError: Cannot read properties of null (reading 'markForCheck')
    at AsyncPipe2._updateLatestValue (http://angular-ci.local:9876/base/dist/legacy-test-bundle.spec.js?49174f830d8743d5c8a9551b77550b859b934291:51947:19)
```

This is caused by the fact that `ref` in `AsyncPipe` is initialized with a value of `null` which causes `_updateLatestValue` to fail since it is not expected to be `null`.

This change ensures that a `ref` is always provided and that all subscriptions are disposed off after each test.

PR Close #49433