Need ability for no site prefix in dart url

In dart.js, when the script attempts to build the dart url, one of the steps is that it prefixes a tag.site value with a "." at the beginning of that value. In my particular issue, tag.site is a NULL value, basically causing this script to add the "." with nothing after it, resulting in a "./" value. This causes the ads to not render correctly for this case.

I've added a conditional statement to dart.js that checks for a null value of tag.site before adds this string into the equation. This keeps all original functionality of the module while at the same time allowing for unique cases to omit the value if needed.

Attached it the patch file with the condition implemented.