Set lock_version to default to 0, non-null

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We've had to monkey patch optimistic locking to handle non-null values in gitlab-ce!7867. We had to revert it in gitlab-ce!5245 because it took more than an hour to migrate issues and merge requests.

I think we want to make this the default, but if it takes significant downtime we will have to reconsider.

/cc: @godfat, @vsizov, @yorickpeterse