fix PHP 7.2 compatibility
fix PHP 7.2 compatibility

* the `phpdocumentor/type-resolver` package was not PHP 7.2 compatible
  before release 0.2.1 (see see phpDocumentor/TypeResolver@e224fb2)
* the validator must not call `get_class()` if no object but a class
  name was passed to the `validatePropertyValue()` method

symfony/framework-bundle