[HttpKernel] Ensure the storage exists before purging it in ProfilerTest

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11319
| License       | MIT
| Doc PR        | None
In Symfony code we usually opt for [Yoda conditions](http://en.wikipedia.org/wiki/Yoda_conditions), so this line should be:

```php
if (null !== $this->storage) {
```