[HttpKernel] Updated TraceableEventDispatcher.php

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Currently, if no listeners are registered for an dispatched event, the StopwatchEvent "$eventName.'.loading'" is never being stopped.