Fix default value for SELENIUM_DRIVER_ARGUMENTS as empty list

[laggardkernel]

Settings.get() default value is None. The default value for SELENIUM_DRIVER_ARGUMENTS should be an empty list to support for loop.

class SeleniumMiddleware:
    def __init__(self, driver_name, driver_executable_path,
        browser_executable_path, command_executor, driver_arguments):
        ...
        for argument in driver_arguments:
            driver_options.add_argument(argument)