Fix default value of driver_arguments

`driver_arguments` is a list. With this patch, if `SELENIUM_DRIVER_ARGUMENTS`
is unset the default value is an empty list instead of `None`.

Author: nfk

scrapy_selenium/middlewares.py

@@ -73,7 +73,7 @@ def from_crawler(cls, crawler):
         driver_executable_path = crawler.settings.get('SELENIUM_DRIVER_EXECUTABLE_PATH')
         browser_executable_path = crawler.settings.get('SELENIUM_BROWSER_EXECUTABLE_PATH')
         command_executor = crawler.settings.get('SELENIUM_COMMAND_EXECUTOR')
-        driver_arguments = crawler.settings.get('SELENIUM_DRIVER_ARGUMENTS')
+        driver_arguments = crawler.settings.get('SELENIUM_DRIVER_ARGUMENTS', [])
 
         if driver_name is None:
             raise NotConfigured('SELENIUM_DRIVER_NAME must be set')