Fixed css value of 0 (Number)

I ran into a problem where some existing web code that now had to be run on the server-side would try to set css values to the `Number 0` (vs the more correct way of a `String '0'`). I ran into this issue with `fill-opacity` and `stroke-opacity`SVG properties, but this could affect any property. Needless to say, this works fine in browsers. 

The previous code here would evaluate `0` as `false`, which would result in this function returning an empty string as the css value. This new value check for undefined should handle both undefined and nulls, while letting all other values pass through untouched.