Fixed EZP-22921: eZPublish installation shows undefined variable in erro...

...r.log

Fixed ini reading functions, and ensure that the Extensions not set case is handled.

https://jira.ez.no/browse/EZP-22921
What would you think about replacing this with `is_array()`? It's a bit more explicit and would cover the (I don't know how probable) case that `extensionList` is `null` or `0`.