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

@@ -31,6 +31,7 @@
         "phpdocumentor/reflection-docblock": "^3.0|^4.0"
     },
     "conflict": {
+        "phpdocumentor/type-resolver": "<0.2.1",
         "symfony/dependency-injection": "<3.2",
         "symfony/property-access": ">=3.0,<3.0.4|>=2.8,<2.8.4",
         "symfony/property-info": "<3.1",