[Validator] fixed: Expressions always valid for null values

The ExpressionValidator was incorrectly skipping validation of null or
empty string values.
feature #11709 [Validator] Expression validator now processes null values (tommygnr)

This PR was merged into the 2.6-dev branch.

Discussion
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[Validator] Expression validator now processes null values

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes(minor)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

The ExpressionValidator was incorrectly skipping validation of null or empty string values.

For example the following was (incorrectly) considered valid if hairColour is null because the validator was skipped
```php
<?php

namespace Acme\DemoBundle\Model\Person;

use Symfony\Component\Validator\Constraints as Assert;

class Person
{
    private $hasHair;

    /**
     * @Assert\Expression(
     *     "!(this.hasHair() and value == null)",
     *     message="If you have hair you must pick its colour!"
     * )
     */
    private $hairColour;
}
```

This is a follow on from #11590 but is targeted against master as the BC break introduced was considered undesirable for currently released versions of symfony.

I will squash and create a documentation PR once there is consensus that this is ready to be merged.

Commits
-------

580e1a7 [Validator] fixed: Expressions always valid for null values