Remove {mollom} schema default values for reason and languages

Issue Summary

Spin-off from #1250614: Allow to protect forms without text fields with text analysis without resorting to CAPTCHA (e.g., user registration form)

Like all other Mollom response values, the default value should be NULL, so we are able to differentiate between a value and no value.

Apparently, Mollom always responds with an empty reason string, but that's a bug that needs to be fixed elsewhere.