pg: Make date_neg_utc_offset is not null if date is not null.

Fixes a regression in 038a219f, as it made converters expect a boolean.

We stopped writing this kind of nulls since we started using model objects for insertions, but didn't migrate existing data.

This commit migrates these nulls to false, then adds a constraint to enforce it in the future.

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Migrated from D3802 (view on Phabricator)