[HttpKernel] Updated TraceableEventDispatcher.php
merged branch janschoenherr/patch-4 (PR #7365)

This PR was squashed before being merged into the 2.2 branch (closes #7365).

Commits
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d9ee869 [HttpKernel] Updated TraceableEventDispatcher.php

Discussion
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[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.