Protect HTML::Scrubber from being fed an undefined variable.

This can be more elegantly done using the // construction but that
breaks perl 5.8   -- so we'll do it this way for now.
Merge branch 'release/2.7' of https://github.com/openwebwork/webwork2 into release/2.7
Merge pull request #75 from mgage/release/2.7

Fix bug in using HTML::Scrubber