Fix for null values in assosiated properties when using UniqueEntityValidator
bug #9979 [Doctrine Bridge][Validator] Fix for null values in assosiated properties when using UniqueEntityValidator (vpetrovych)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #9979).

Discussion
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[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.

Commits
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bf368bc [Doctrine Bridge][Validator] Fix for null values in assosiated properties when using UniqueEntityValidator