[WebServerBundle] fixed server:log when Monolog is not installed

src/Symfony/Bundle/WebServerBundle/Command/ServerLogCommand.php

@@ -29,10 +29,24 @@ class ServerLogCommand extends Command
     private $el;
     private $handler;
 
+    public function isEnabled()
+    {
+        if (!class_exists(ConsoleFormatter::class)) {
+            return false;
+        }
+
+        return parent::isEnabled();
+    }
+
     protected function configure()
     {
+        $this->setName('server:log');
+
+        if (!class_exists(ConsoleFormatter::class)) {
+            return;
+        }
+
         $this
-            ->setName('server:log')
             ->setDescription('Start a log server that displays logs in real time')
             ->addOption('host', null, InputOption::VALUE_REQUIRED, 'The server host', '0:9911')
             ->addOption('format', null, InputOption::VALUE_REQUIRED, 'The line format', ConsoleFormatter::SIMPLE_FORMAT)