don't call EntityManager::initializeObject() with scalar values

Calling initializeObject() with a scalar value (e.g. the id of the
associated entity) was a no-op call with Doctrine ORM 2 where no type
was set with the method signature. The UnitOfWork which was called
with the given argument ignored anything that was not an InternalProxy
or a PersistentCollection instance.

Calls like these now break with Doctrine ORM 3 as the method's argument
is typed as object now.

Author: xabbuh

src/Symfony/Bridge/Doctrine/Validator/Constraints/UniqueEntityValidator.php

@@ -104,7 +104,7 @@ public function validate($entity, Constraint $constraint)
 
             $criteria[$fieldName] = $fieldValue;
 
-            if (null !== $criteria[$fieldName] && $class->hasAssociation($fieldName)) {
+            if (\is_object($criteria[$fieldName]) && $class->hasAssociation($fieldName)) {
                 /* Ensure the Proxy is initialized before using reflection to
                  * read its identifiers. This is necessary because the wrapped
                  * getter methods in the Proxy are being bypassed.