[Doctrine Bridge][Validator] Fix for null values in assosiated properties when using UniqueEntityValidator

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7382
| License       | MIT
| Doc PR        | 

This fixes issue with UniqueEntityValidator when ignoreNull = false and associated property contains null value:
```
Warning: ReflectionProperty::getValue() expects parameter 1 to be object, null given in [...]/symfony2/vendor/doctrine/orm/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php line 670 
```

Here's what docs say about ignoreNull:

> If this option is set to true, then the constraint will allow multiple entities to have a null value for a field without failing validation. If set to false, only one null value is allowed - if a second entity also has a null value, validation would fail.