Allow classes without contructors

Author: fredden

setup/src/Magento/Setup/Module/Di/Code/Reader/Decorator/Area.php

@@ -50,7 +50,7 @@ public function getList($path)
     {
         $classes = [];
         foreach ($this->classesScanner->getList($path) as $className) {
-            $classes[$className] = $this->classReaderDecorator->getConstructor($className);
+            $classes[$className] = (array) $this->classReaderDecorator->getConstructor($className);
         }
 
         return $classes;

setup/src/Magento/Setup/Module/Di/Definition/Collection.php

@@ -91,6 +91,6 @@ public function getInstancesNamesList()
      */
     public function hasInstance($instanceName)
     {
-        return isset($this->definitions[$instanceName]);
+        return array_key_exists($instanceName, $this->definitions);
     }
 }