[Notifier] Add correct tags for NullTransportFactory

[jschaedl]

Q	A
Branch?	5.0
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	-
License	MIT
Doc PR	-

I tried to disable the delivery of notifications in dev environment with the following configuration:

framework:
    notifier:
        chatter_transports:
            slack: 'null://null'
        texter_transports:
            twilio: 'null://null'
        channel_policy:
            urgent: ['chat/slack', 'sms/twilio']
            high: ['email']
            medium: ['email']
            low: ['email']

While sending the notification like this:

$notification = (new Notification())
            ->subject('Test subject')
            ->importance(Notification::IMPORTANCE_URGENT)
            ->content('Test content')
;

$this->notifier->send($notification);

I got an UnsupportedSchemeException: The "null" scheme is not supported.

After some digging I figured out that this Exception occurred because the NullTransportFactory was not tagged with the chatter.transport_factory and texter.transport_factory tags. Which is the reason the NullTransportFactory was not injected in the Transport class and so the NullTransport couldn't be used.

This PR should fix this Bug.

[nicolas-grekas]

Thank you @jschaedl.