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%; 
```
Revert "Bail if we don't find a matching code"

This reverts commit d2d9b06dcb9acfff1401a1e600fbad7c3df03370.

Ended up causing issues on some VIP sites and the original problem this
was tackling ended up being a different problem altogether.
Fix Strict 'pass by reference' warning by not calling array_shift() on the direct result of array_shift()

Strict Standards: Only variables should be passed by reference in
path/ad-code-manager/ad-code-manager.php on line 1003