Fix Rails optimistic locking when value in column is NULL

We've had issues with optimistic locking in issues and MRs because our lock_version columns can contain NULLs. gitlab-com/gl-infra/production#821 (closed) is one of these, and there are some links from there to previous cases.

https://github.com/rails/rails/pull/26050 was intended to fix this, but didn't. @engwan pointed out a bug here: https://github.com/rails/rails/blob/v5.1.7/activerecord/lib/active_record/locking/optimistic.rb#L83

previous_lock_value = read_attribute_before_type_cast(locking_column)

This is reading the value from the attribute on the model, which is already dirtied. So if lock_version was NULL in the DB, and the user passed 0, this would return 0, not NULL. The update would then fail as the column doesn't have 0: https://github.com/rails/rails/blob/v5.1.7/activerecord/lib/active_record/locking/optimistic.rb#L91