fix: Set messenger.transports if config has not empty transports value

[ahonymous]

These changes fix the issue in BrefMessengerExtension::prepand() for applications with multiple configs in different environments.

In BrefMessengerExtension.php line 25:

Warning: Undefined array key "transports"

Example:

configs
|__ packages
    |__dev
    |    |__messenger.yaml
    |__prod
    |    |__messenger.yaml
    |__messenger.yaml     

# configs/messenger.yaml
...
framework:
    messenger:
        transports:
            default_bus:
                <transport-config>
        routing:
            App\Message\Message: default_bus

# configs/dev/messenger.yaml
...
framework:
    messenger:
        buses:
            <dev-buses-config>

# configs/prod/messenger.yaml
...
framework:
    messenger:
        buses:
            <prod-buses-config>

[mnapoli]

Beautiful work thank you!