[BUGFIX] Allow DEFAULT NULL for VARCHAR

The database schema generated by `DefaultTcaSchema` lacks the ability to
generate `VARCHAR` fields with `NULL` as their default value.

The code is adjusted to generate `DEFAULT NULL` for input fields that do
not exceed the maximum length of 255 characters.

Since this code adjusts existing database schema with potentially
negative impact, this patch is aimed for main only.

Resolves: #106656
Releases: main
Change-Id: Icba421059819ddd6cfdb7bf55c3b80d17dc0b7bc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/89299
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>

Author: andreaskienast

typo3/sysext/core/Classes/Database/Schema/DefaultTcaSchema.php

@@ -803,7 +803,7 @@ protected function enrichSingleTableFieldsFromTcaColumns(array $tables): array
                             Types::STRING,
                             [
                                 'length' => $length,
-                                'default' => '',
+                                'default' => $nullable ? null : '',
                                 'notnull' => !$nullable,
                             ]
                         );

typo3/sysext/core/Documentation/Changelog/14.0/Important-106656-AllowDEFAULTNULLForVarcharFields.rst

@@ -0,0 +1,60 @@
+..  include:: /Includes.rst.txt
+
+..  _important-106656-1746525351:
+
+============================================================
+Important: #106656 - Allow `DEFAULT NULL` for varchar fields
+============================================================
+
+See :issue:`106656`
+
+Description
+===========
+
+In TCA, if an input field is configured to be nullable via
+:php:`'nullable' => true`, the database migration now respects this and creates
+new or updates existing fields with `DEFAULT NULL`.
+
+In Extbase, this may cause issues if properties and their accessor methods are
+not properly declared to be nullable, therefore this change is introduced to
+TYPO3 v14 only.
+
+Example:
+
+..  code-block:: php
+    :caption: Example properly implementing a nullable property
+
+    <?php
+
+    declare(strict_types=1);
+
+    namespace Vendor\myExtension\Domain\Model;
+
+    use TYPO3\CMS\Extbase\DomainObject\AbstractEntity;
+
+    class MyExtbaseEntity extends AbstractEntity
+    {
+        protected ?string $title;
+
+        public function getTitle(): ?string
+        {
+            return $this->title;
+        }
+
+        public function setTitle(?string $title): void
+        {
+            $this->title = $title;
+        }
+    }
+
+As stated above, this automatic detection is not provided in TYPO3 versions
+older than 14.0. Using `DEFAULT NULL` can be enforced via an extension's
+:file:`ext_tables.sql` instead:
+
+..  code-block:: sql
+
+    CREATE TABLE tx_myextension_table (
+        title varchar(255) DEFAULT NULL
+    );
+
+..  index:: Database, ext:core

typo3/sysext/core/Tests/Unit/Database/Schema/DefaultTcaSchemaTest.php

@@ -1906,7 +1906,7 @@ public function enrichAddsInputAndConsidersNullable(): void
             Type::getType('string'),
             [
                 'length' => 255,
-                'default' => '',
+                'default' => null,
                 'notnull' => false,
             ]
         );

typo3/sysext/extbase/Tests/Functional/Fixtures/Extensions/blog_example/Classes/Domain/Model/Blog.php

@@ -82,7 +82,12 @@ public function __construct()
         $this->logo = new ObjectStorage();
     }
 
-    public function getSubtitle(): string
+    public function setSubtitle(?string $subtitle): void
+    {
+        $this->subtitle = $subtitle;
+    }
+
+    public function getSubtitle(): ?string
     {
         return $this->subtitle;
     }
@@ -181,9 +186,4 @@ public function getAdministrator(): Administrator|LazyLoadingProxy|null
     {
         return $this->administrator;
     }
-
-    public function setSubtitle(?string $subtitle): void
-    {
-        $this->subtitle = $subtitle;
-    }
 }