backport (dpvc): Made str_cmp accept blank answers again. Fixes bug #1123.
Note: if you use blank answers, the "n of the questions remain
unanswered" message will tell you how many of the answers should be
left blank. It is probably worth working out a method of having blank
entries that are only marked correct when all the other answers are
correct (this would take a special grader; I have this as part of the
weighted grader, but it is not common knowledge).