[Validator] fixed: Expressions always valid for null values

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| 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;
}
```
Could you please add tests which explicitly treat the null and empty string scenarios? We need to document that this is an actual feature of the constraint.