Finish the method definition below.
Given an hash person with keys first_name and last_name, the method full_name should return a string which combines first_name and last_name separated by a space.
Note:
- If the first_name doesn't exist, return just the last_name as the full name
- If the last_name doesn't exist, return just the first_name as the full name
- If both first_name and last_name don't exist, return
"" as the full name.