make pref-file_exists condition more explicit

Author: TomasVotruba

src/Types/ContextFactory.php

@@ -47,7 +47,7 @@ public function createFromReflector(\Reflector $reflector)
         $fileName = $reflector->getFileName();
         $namespace = $reflector->getNamespaceName();
 
-        if ($fileName && file_exists($fileName)) {
+        if (is_string($fileName) && file_exists($fileName)) {
             return $this->createForNamespace($namespace, file_get_contents($fileName));
         }