feat(Mailer): add "null" SMTP transport mode

== Goal

Allow disabling mail delivery altogether.

== Usecase

If mails ought to be send by other means than rendering messages from
templates and sending them via SMTP-like protocols.

Example: listening to specific Nextcloud events and pass parameters to
a centralized (i.e. REST-based) API that sends e-mails.

Signed-off-by: Thomas Lehmann <t.lehmann@strato.de>

config/config.sample.php

@@ -521,7 +521,7 @@
 'mail_smtpdebug' => false,
 
 /**
- * Which mode to use for sending mail: ``sendmail``, ``smtp`` or ``qmail``.
+ * Which mode to use for sending mail: ``sendmail``, ``smtp``, ``qmail`` or ``null``.
  *
  * If you are using local or remote SMTP, set this to ``smtp``.
  *
@@ -531,6 +531,9 @@
  * For ``qmail`` the binary is /var/qmail/bin/sendmail, and it must be installed
  * on your Unix system.
  *
+ * Use the string ``null`` to send no mails (disable mail delivery). This can be
+ * useful if mails should be sent via APIs and rendering messages is not necessary.
+ *
  * Defaults to ``smtp``
  */
 'mail_smtpmode' => 'smtp',

lib/private/Mail/Mailer.php

@@ -27,6 +27,7 @@
 use Symfony\Component\Mailer\Exception\TransportExceptionInterface;
 use Symfony\Component\Mailer\Mailer as SymfonyMailer;
 use Symfony\Component\Mailer\MailerInterface;
+use Symfony\Component\Mailer\Transport\NullTransport;
 use Symfony\Component\Mailer\Transport\SendmailTransport;
 use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport;
 use Symfony\Component\Mailer\Transport\Smtp\Stream\SocketStream;
@@ -256,6 +257,9 @@ protected function getInstance(): MailerInterface {
 		}
 
 		switch ($this->config->getSystemValueString('mail_smtpmode', 'smtp')) {
+			case 'null':
+				$transport = new NullTransport();
+				break;
 			case 'sendmail':
 				$transport = $this->getSendMailInstance();
 				break;

tests/lib/Mail/MailerTest.php

@@ -114,6 +114,26 @@ public function testGetSendmailInstanceSendMailQmail($sendmailMode, $binaryParam
 		$this->assertEquals($sendmail, self::invokePrivate($this->mailer, 'getSendMailInstance'));
 	}
 
+	public function testEventForNullTransport(): void {
+		$this->config
+			->expects($this->exactly(1))
+			->method('getSystemValueString')
+			->with('mail_smtpmode', 'smtp')
+			->willReturn('null');
+
+		$message = $this->createMock(Message::class);
+		$message->expects($this->once())
+			->method('getSymfonyEmail')
+			->willReturn((new Email())->to('foo@bar.com')->from('bar@foo.com')->text(''));
+
+		$event = new BeforeMessageSent($message);
+		$this->dispatcher->expects($this->once())
+			->method('dispatchTyped')
+			->with($this->equalTo($event));
+
+		$this->mailer->send($message);
+	}
+
 	public function testGetInstanceDefault(): void {
 		$this->config
 			->method('getSystemValue')