Fixed issue where value is '0'

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'). 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.