Fix types of Console\Command properties (#37495)

`Command::$description` and `Command::$help` are never null,
because they are set via `Command::setDescription()` and 
`Command::setHelp()` respectively in the constructor.

Author: caugner

src/Illuminate/Console/Command.php

@@ -38,14 +38,14 @@ class Command extends SymfonyCommand
     /**
      * The console command description.
      *
-     * @var string|null
+     * @var string
      */
     protected $description;
 
     /**
      * The console command help text.
      *
-     * @var string|null
+     * @var string
      */
     protected $help;