Highscore Menu Fix

Handles occasions where there are fewer than 10 highscores.

Before it would display 0 value highscores when there were none
recorded. It would also consider a 0 highscore (which the user has when
coming directly from the Main Menu) as greater than a null highscore.

NOTE: someone should fix the Background for the MainGameplay and
HighScoreMenu scenes because they haven't been adapted to work with the
new Background-handling system.