Finish the method definition below.
vacation_details is a hash containing details of a vacation. Given vacation_details, and day and year as integer arguments, the method should add a start_date key in the hash with a date in the format of "Fri Dec 07 2018" as per the year and day inputs passed to it, and month as December. If the start_date key is already present in the hash, the method should not update it.
The method should return the updated vacation_details.
Note: Try using Logical nullish assignment.