[applets/kickoff] Always instantiate KService::Ptr member
Code uses m_service unconditionally in many branches, it is exposed as public API. Given KService is a data class with an isValid method, it's clearer to ensure that KService::Ptr always returns a valid object which in turn can then be valid rather than having two paths for the same goal.
This can be relevant in a situation where an entry using the "preferred" schema references an entry that doesn't exist.