EZP-21829: Avoid notice when overriding a nonexisting variable

When trying to override an INI setting in an Unit Test which doesn't exist yet in an INI file, `ezpINIHelper` triggers a notice that the variable doesn't exist.

This can happen if a variable is not present in a default INI file, but has been set in an override INI in `settings/override` - as the standard `settings/override` dir is disabled during tests, this can lead to this behavior.

https://jira.ez.no/browse/EZP-21829

Cheers
:octocat: Jérôme