bug #31336 [DoctrineBridge] do not choke on not existing mapping field (xabbuh)

This PR was merged into the 4.3-dev branch.

Discussion
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[DoctrineBridge] do not choke on not existing mapping field

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 31239
| License       | MIT
| Doc PR        |

Commits
-------

9a9134f4e4 do not choke on not existing mapping field

src/Symfony/Bridge/Doctrine/Tests/Fixtures/BaseUser.php

@@ -17,6 +17,8 @@ class BaseUser
      */
     private $username;
 
+    private $enabled;
+
     /**
      * BaseUser constructor.
      *

src/Symfony/Bridge/Doctrine/Tests/Resources/orm/BaseUser.orm.xml

@@ -6,6 +6,7 @@
 
     <mapped-superclass name="Symfony\Bridge\Doctrine\Tests\Fixtures\BaseUser">
         <field name="username" column="username" type="string" length="120" />
+        <field name="enabled" type="boolean" />
     </mapped-superclass>
 
 </doctrine-mapping>

src/Symfony/Bridge/Doctrine/Validator/DoctrineLoader.php

@@ -72,7 +72,7 @@ final class DoctrineLoader implements LoaderInterface
                 $metadata->addConstraint(new UniqueEntity(['fields' => $mapping['fieldName']]));
             }
 
-            if (null === $mapping['length']) {
+            if (null === ($mapping['length'] ?? null)) {
                 continue;
             }