The change in MDL-51090 broke manually commenting on questions for which
no mark is given (max mark == 0). This fixes it, with unit tests.
Thanks to Nick Phillips for the original suggestion of the fix.
1/ type/match/tests/walkthrough_test.php - tests are failing randomly, looks like some weird randomisation is going on there - see TODOs
2/ type/multianswer/tests/upgradelibnewqe_test.php contains invalid expected value - see TODO