getAdditionalPropertyNames cause a warning if null (#75)

* bugfix - getAdditionalPropertyNames warning if null

* return empty array and improve performance

Co-Authored-By: Viacheslav Poturaev <vearutop@users.noreply.github.com>

src/Structure/ObjectItemTrait.php

@@ -51,6 +51,9 @@ public function addAdditionalPropertyName($name)
      */
     public function getAdditionalPropertyNames()
     {
+        if (null === $this->__additionalPropertyNames) {
+            return [];
+        }
         return array_keys($this->__additionalPropertyNames);
     }
 
@@ -155,4 +158,4 @@ protected function getFromRefPath() {
         }
         return $this->__refPath;
     }
-}
+}