[8.x] Guess database factory model by default (#39310)

* Guess database factory model by default

* Fixes property `$model` type

Author: nunomaduro

src/Illuminate/Database/Console/Factories/stubs/factory.stub

@@ -3,17 +3,9 @@
 namespace {{ factoryNamespace }};
 
 use Illuminate\Database\Eloquent\Factories\Factory;
-use {{ namespacedModel }};
 
 class {{ factory }}Factory extends Factory
 {
-    /**
-     * The name of the factory's corresponding model.
-     *
-     * @var string
-     */
-    protected $model = {{ model }}::class;
-
     /**
      * Define the model's default state.
      *

src/Illuminate/Database/Eloquent/Factories/Factory.php

@@ -24,7 +24,7 @@ abstract class Factory
     /**
      * The name of the factory's corresponding model.
      *
-     * @var string
+     * @var string|null
      */
     protected $model;