MQE-477: Generate tests throws warning when using action groups in merged tests.

- Changed setter of argData[argument] to not error on null arguments.

Author: KevinBKozan

src/Magento/FunctionalTestingFramework/Test/Util/ActionObjectExtractor.php

@@ -107,7 +107,7 @@ private function processActionGroupArgs($actionAttributeData)
             }
 
             $actionAttributeArgData[self::ACTION_GROUP_ARGUMENTS][$attributeDataKey] =
-                $attributeDataValues[self::ACTION_GROUP_ARG_VALUE];
+                $attributeDataValues[self::ACTION_GROUP_ARG_VALUE] ?? null;
         }
 
         return $actionAttributeArgData;