Bail if we don't find a matching code

In some cases, get_matching_ad_code will not return an ad because nothing's been registered against that tag. In some cases, we can end up with invalid ad code being output, because that passes the script url validation (via validate_script_url).

```
google_ad_client = "%publisher_id%"; 
google_ad_slot = "%tag_id%"; 
google_ad_width = %width%; 
google_ad_height = %height%; 
```