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

Author: xabbuh

composer.json

@@ -61,7 +61,7 @@
     },
     "conflict": {
         "phpdocumentor/reflection-docblock": "<3.0",
-        "phpdocumentor/type-resolver": "<0.2.0",
+        "phpdocumentor/type-resolver": "<0.2.1",
         "phpunit/phpunit": "<4.8.35|<5.4.3,>=5.0",
         "symfony/asset": "<3.3",
         "symfony/console": "<3.3",