Fix "warning: instance variable @browser not initialized"

Followup to 0009699.

Author: eugeneius

actionpack/lib/action_dispatch/system_testing/driver.rb

@@ -13,6 +13,8 @@ def initialize(name, **options, &capabilities)
           require "selenium/webdriver"
           @browser = Browser.new(options[:using])
           @browser.preload
+        else
+          @browser = nil
         end
       end