Support `is_null()` in `TernaryToNullCoalescingRector`

# Feature Request

The `TernaryToNullCoalescingRector` already refactors `$value === null` and `isset($value)`, I would also propose refactor `is_null($value)` to `??`

```diff
-$value === null ? 10 : $value;
+$value ?? 10;
```

```diff
-isset($value) ? $value : 10;
+$value ?? 10;
```
 
## Diff 

New:

```diff
-is_null($value) ? 10 : $value;
+$value ?? 10;
```
