False-positive result on add nullable type in final method of abstract class

Example:

```php
abstract class X {
	final public function test(string $var): self
}
```

changed to:

```php
abstract class X {
	final public function test(?string $var): self
}
```

result will be:

```
[BC] CHANGED: The parameter $var of X#test() changed from string to ?string
```

Real example: https://github.com/yiisoft/html/pull/78