bug [mailer] fix EsmtpTransport variable $code definition

the variable $code defined in the foreach loop, if there is no authenticators,
then the $code will not be defined and therefore the following TransportException
gives a PHP error.

the use-case defined as a unit test in EsmtpTransportTest class.

Tests/Transport/Smtp/EsmtpTransportTest.php

@@ -192,6 +192,28 @@ public function testSetAuthenticators()
             $stream->getCommands()
         );
     }
+
+    public function testConstructorWithEmptyAuthenticator()
+    {
+        $stream = new DummyStream();
+        $transport = new EsmtpTransport(stream: $stream);
+        $transport->setUsername('testuser');
+        $transport->setPassword('p4ssw0rd');
+        $transport->setAuthenticators([]); // if no authenticators defined, then there needs to be a TransportException
+
+        $message = new Email();
+        $message->from('[email protected]');
+        $message->addTo('[email protected]');
+        $message->text('.');
+
+        try {
+            $transport->send($message);
+            $this->fail('Symfony\Component\Mailer\Exception\TransportException to be thrown');
+        } catch (TransportException $e) {
+            $this->assertStringStartsWith('Failed to find an authenticator supported by the SMTP server, which currently supports: "plain", "login", "cram-md5", "xoauth2".', $e->getMessage());
+            $this->assertEquals(504, $e->getCode());
+        }
+    }
 }
 
 class CustomEsmtpTransport extends EsmtpTransport

Transport/Smtp/EsmtpTransport.php

@@ -184,6 +184,7 @@ private function handleAuth(array $modes): void
             return;
         }
 
+        $code = null;
         $authNames = [];
         $errors = [];
         $modes = array_map('strtolower', $modes);
@@ -192,7 +193,6 @@ private function handleAuth(array $modes): void
                 continue;
             }
 
-            $code = null;
             $authNames[] = $authenticator->getAuthKeyword();
             try {
                 $authenticator->authenticate($this);