Don't crash with missing optional args to css

On Sat, Oct 04, 2014 at 15:17:47 -0700, Dylan Simon wrote:
> Understood.  When I have time I may look into the WebKitWebExtension
> stuff.  As I'm sure you know webkitgtk has removed webkit1, so I'm
> motivated to get off it.

Same here.

> (I use uzbl-core but not the python front-end.)

Out of curiosity, what do you use instead? If it turns out "no one" is
using the stock daemon, maybe it should be replaced?