Use null instead of ini values for mysqli connection, when running HHVM.

HHVM was returning a boolean from ini_get() for some reason, which caused the page to turn blank.
Now it is using null instead, only when running HHVM.
I don't know if you want to keep on using the ini_values for normal php, because I think it does use the ini_values now too.
I'm not even sure if you want to add an exception for when running HHVM, but I thought it's worth a PR.