[Validator] Made it possible (again) to pass a class name to validatePropertyValue()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11139
| License       | MIT
| Doc PR        | -

In the 2.4 API it was possible to do both:

```php
$validator->validatePropertyValue($object, 'propertyName', $myValue);
$validator->validatePropertyValue('\Vendor\Namespace\ClassName', 'propertyName', $myValue);
```

In the 2.5 API, the second case was not supported anymore. This is fixed now.

Together with the fix comes also a small change (also in the 2.4 API) which I'll demonstrate with a code snippet:

```php
$metadata->addPropertyConstraint('ClassName', 'propertyName', new Callback(
    function ($value, $context) {
        var_dump($context->getRoot());
        var_dump($context->getPropertyPath());
    }
));

$validator->validatePropertyValue('ClassName', 'propertyName', 'foobar');
```

Before this PR, the output would be:

```
string(9) "ClassName"
string(12) "propertyName"
```

This doesn't make a lot of sense, because usually the following condition holds during validation:

```php
'' === $context->getPropertyPath() || $value === $propertyAccessor->getValue($context->getRoot(), $context->getPropertyPath())
```

which obviously cannot work if root is a class name. Thus I changed the root and property path to become:

```
string(6) "foobar"
string(0) ""
```

With this change, the condition holds also in this case.
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