Khan/khan-exercises — pull request #33803 by xymostech (2012-11-15 20:04:49+00:00)
Title: Fix fallbacks in answer types

Because `undefined + ""` produces the string `"undefined"`, fallbacks
did not accurately show that they were not available. This causes
some/all of the problems to inaccurately never report that an answer was
not entered. This should fix that.

@beneater