minor #39905 [Notifier] Check for MercureBundle in MercureTransportFactory (mtarld)

This PR was merged into the 5.3-dev branch.

Discussion
----------

[Notifier] Check for MercureBundle in MercureTransportFactory

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

https://github.com/symfony/symfony/pull/39903 has removed the check for `MercureBundle` from the `FrameworkExtension`.
The following PR is re-adding that check but in the `MercureTransportFactory` class.

Commits
-------

49bbbc1ed5 [Notifier] Check for MercureBundle in Factory