Make Page::reference nullable to route null through NotBlank validation

Symfony 8.2 improved how null-for-typed-string is handled during
deserialization: instead of a raw TypeError message, it now produces
a proper validation violation. Rather than rely on Symfony's internal
error message (which changed between 8.1 and 8.2), make the reference
column nullable at both the PHP and ORM level so null is accepted during
deserialization and our explicit NotBlank constraint fires consistently
across all Symfony versions.

The "incorrect data types" scenario (which tested the raw TypeError text)
is replaced by a null-reference example in the existing violations outline,
testing against our own "Please enter a reference." message.

Author: silverbackdan

features/main/page.feature

@@ -102,21 +102,9 @@ Feature: Page resources
       | layout             | reference | propertyPath       | message                                    | uiComponent    |
       | null               | home      | layout             | Please specify a layout.                   | myComponent    |
       | resource[layout]   |           | reference          | Please enter a reference.                  | myComponent    |
+      | resource[layout]   | null      | reference          | Please enter a reference.                  | myComponent    |
       | resource[layout]   | home      | uiComponent        | Please specify a UI component.             |                |
 
-  @loginUser
-  Scenario Outline: The page resource returns errors on incorrect data types
-    Given there is a Layout
-    When I send a "POST" request to "/_/pages" with data:
-      | layout    | reference   | isTemplate |
-      | <layout>  | <reference> | false      |
-    Then the response status code should be 422
-    And the JSON should be valid according to the schema file "error.schema.json"
-    And the JSON node "description" should be equal to the string '<message>'
-    Examples:
-      | layout             | reference | message                                                               |
-      | resource[layout]   | null      | The type of the "reference" attribute must be "string", "NULL" given. |
-
   @loginAdmin
   Scenario: I can delete a page
     Given there is a Page

src/Entity/Core/Page.php

@@ -45,9 +45,9 @@ class Page extends AbstractPage
     #[Groups(['Route:manifest:read'])]
     public ?Layout $layout;
 
-    #[ORM\Column(unique: true)]
+    #[ORM\Column(unique: true, nullable: true)]
     #[Assert\NotBlank(message: 'Please enter a reference.')]
-    public string $reference;
+    public ?string $reference = null;
 
     #[ORM\Column(name: 'is_template')]
     #[Assert\NotNull(message: 'Please specify if this page is a template or not.')]