do not validate empty values

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

@@ -62,6 +62,10 @@ class UniqueEntityValidator extends ConstraintValidator
             throw new ConstraintDefinitionException('At least one field has to be specified.');
         }
 
+        if (null === $entity) {
+            return;
+        }
+
         if ($constraint->em) {
             $em = $this->registry->getManager($constraint->em);
 

src/Symfony/Component/Security/Core/Validator/Constraints/UserPasswordValidator.php

@@ -39,6 +39,10 @@ class UserPasswordValidator extends ConstraintValidator
             throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\UserPassword');
         }
 
+        if (null === $password || '' === $password) {
+            return;
+        }
+
         $user = $this->tokenStorage->getToken()->getUser();
 
         if (!$user instanceof UserInterface) {

src/Symfony/Component/Validator/Constraints/UrlValidator.php

@@ -48,7 +48,7 @@ class UrlValidator extends ConstraintValidator
             throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Url');
         }
 
-        if (null === $value) {
+        if (null === $value || '' === $value) {
             return;
         }