This should be a transparent change but I thought it would have been better
to keep all of you informed in case of unintended regressions or
side-effects. The context of the alter tables is in T167162 but the TL;DR
is that we need nullable attributes across all the EL tables (except fields
like id, uuid and timestamp) to be able to sanitize data with our new
eventlogging_cleaner script (https://phabricator.wikimedia.org/T156933).
Please let me know if you encounter any issue with this change.