UPGRADE FROM 2.5 to 2.6

Validator

The internal method setConstraint() was added to Symfony\Component\Validator\Context\ExecutionContextInterface. With this method, the context is informed about the constraint that is currently being validated.

If you implement this interface, make sure to add the method to your implementation. The easiest solution is to just implement an empty method:
```
public function setConstraint(Constraint $constraint)
{
}
```
Prior to 2.6 Symfony\Component\Validator\Constraints\ExpressionValidator would not execute the Expression if it was attached to a property on an object and that property was set to null or an empty string.

To emulate the old behaviour change your expression to something like this:
```
value == null or (YOUR_EXPRESSION)
```