Don't crash with missing optional args to css

Ah, sounds good. WebKit2 needs a serious rework of things in uzbl to be fully useful. There's a list somewhere, but it is involved. Namely:

[ ] splitting uzbl from the UI thread (hard)
[ ] putting page manipulation commands in a WebKitWebExtension (moderate)
[ ] talking to the extension over a GIOChannel through a private socket (easy)
[ ] shuttling commands from the main uzbl instance over to the extension for the work, then getting the result back (hard)

Basically, it's going to need a concerted effort to get it to work (and WebKit1 support will die in the process) which is why I want a 0.9 release at some point while WebKit2 work forges ahead. I haven't found the time to do it yet though.