EZP-21829: Avoid notice when overriding a nonexisting variable

When trying to override an INI setting which doesn't exist yet in an INI file, ezpINIHelper triggers a notice that the variable doesn't exist.
Merge pull request #812 from jeromegamez/EZP-21829_ezpinihelper

EZP-21829: Avoid notice when overriding a nonexisting variable