See the discussion http://webhost.math.rochester.edu/webworkdocs/discuss/msgReader$3795
In particular, Davide Cervone wrote (toward the end):
OK, I have fixed the Parser to handle emtpy strings. You can use
to get a blank string checker. I have also modified Problem.pm
to not count empty answers that are marked correct as blank answers.
This means that you won't get the "n unanswered questions" warning (nor
the contradictory "all the answers are correct/at least one is NOT
correct" messages). Finally, I modified the MultiPart object to work
with blank answers.
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