Poll upgrade tests fail on SQLite

I hope this time, it's really an easy one :)

Someone the substring_index replacement function for sqlite does get called with empty string during the upgrade process and it doesn't like this because of this line:

$end = strpos($string, $delimiter, $end + 1);

In case of an empty string, this translates to strpos('', '/', 1) and then strpos() complains that it offset is bigger than the string length.

Patch checks if $string is empty and returns an empty string then.

Hm, evil patch... :)