…rvable (#49894)

As described in
#49681 (reply in thread),
if an `Observable` created from a signal with `toObservable` is
subscribed to in a template, it will initially have `null` as the value.
Immediately after the template is done executing, effects are flushed
and this results in the `AsyncPipe` getting a new value before the
`checkNoChanges` pass, resulting in `ExpressionChanged` error.

```
template: '{{obs$ | async}}'
...
obs$ = toObservable(signal(0));
```

Instead, this commit updates the `toObservable` to synchronously emit
the initial value to the Observable stream.

Side note here: We don't exactly encourage this pattern. Instead of
using `AsyncPipe`, the template should just read signals.

PR Close #49894