Fixed logic when retrieving milestones and challenges connected to a user.

Fixed logic when checking if a challenge has expiration date after current date. The logic was switched from deleting the challenge if the expiration date is after current date - to deleting the challenge if the expiration date is before the current date. Did also fix the logic for when retrieving milestones for a user. The logic was switched from initiating a return array with the value null - to initiating a return array with the value of a new empty array. This will fix the issue with null as return value.