Leaderboard default score should be last in ordering

[thoward27]

Describe the bug
The leaderboard's default score is None, it should be whatever value ensures it shows up in the last place.

To Reproduce
Steps to reproduce the behavior:

1. Create a leaderboard question
2. Student's code doesn't complete the test
3. Default value of leaderboardScore may place it at the top of the leaderboard

Expected behavior
Leaderboard default values should put students in last place.