[Messenger] Allow defining an empty sender service

[javiereguiluz]

Symfony version(s) affected: 4.1

Description
While improving Messenger docs (symfony/symfony-docs#9771) we found an example that we don't know if it's going to work or not.

<framework:config>
    <framework:messenger>
        <framework:routing message-class="My\Message\ThatIsGoingToBeSentAndHandledLocally">
            <framework:sender service="amqp" />
            <framework:sender service="" />  <!-- <==== WOULD THIS WORK? -->
        </framework:routing>
    </framework:messenger>
</framework:config>

Original comment from @xabbuh: symfony/symfony-docs#9771 (comment)

[ogizanagi]

Oops! I was trying to answer @xabbuh in symfony/symfony-docs#9771 (comment) but missed symfony/symfony-docs#9771 (comment) afterwards.
This isn't supposed to work anymore AFAIK, as it was replaced by a new send_and_handle option on the routing node instead in #27002

[sroze]

Closing, as @ogizanagi explained.