Allow unset PathItem method

After disabling itemOperation `GET`

```
    itemOperations: [
        'get' => [
            'controller' => ApiPlatform\Core\Action\NotFoundAction::class,
            'read' => false,
            'output' => false,
        ],
    ]
```

there is not nice way to remove that path from OpenApi docs. Now to do
that you have to write:

```
$pathItem = $openApi->getPaths()->getPath('/resource/{id}');
$openApi->getPaths()->addPath(
    '/resource/{id}',
    new Model\PathItem(
        $pathItem->getRef(),
        $pathItem->getSummary(),
        $pathItem->getDescription(),
        null,
        $pathItem->getPut(),
        $pathItem->getPost(),
        $pathItem->getDelete(),
        $pathItem->getOptions(),
        $pathItem->getHead(),
        $pathItem->getPatch(),
        $pathItem->getTrace(),
        $pathItem->getServers(),
        $pathItem->getParameters(),
    )
);
```

After change it will be:

```
$pathItem = $openApi->getPaths()->getPath('/resource/{id}');
$openApi->getPaths()->addPath(
    '/resource/{id}',
    $pathItem->withGet(null)
);
```

Author: d0niek

src/OpenApi/Model/PathItem.php

@@ -138,31 +138,31 @@ public function withDescription(string $description): self
         return $clone;
     }
 
-    public function withGet(Operation $get): self
+    public function withGet(?Operation $get): self
     {
         $clone = clone $this;
         $clone->get = $get;
 
         return $clone;
     }
 
-    public function withPut(Operation $put): self
+    public function withPut(?Operation $put): self
     {
         $clone = clone $this;
         $clone->put = $put;
 
         return $clone;
     }
 
-    public function withPost(Operation $post): self
+    public function withPost(?Operation $post): self
     {
         $clone = clone $this;
         $clone->post = $post;
 
         return $clone;
     }
 
-    public function withDelete(Operation $delete): self
+    public function withDelete(?Operation $delete): self
     {
         $clone = clone $this;
         $clone->delete = $delete;
@@ -186,7 +186,7 @@ public function withHead(Operation $head): self
         return $clone;
     }
 
-    public function withPatch(Operation $patch): self
+    public function withPatch(?Operation $patch): self
     {
         $clone = clone $this;
         $clone->patch = $patch;