db: backfill zero-time node expiry to NULL

Versions before 0.28 persisted a pointer to a zero time.Time as
'0001-01-01 00:00:00+00:00' in nodes.expiry rather than NULL. 0.29
reports those rows as expired. #3197 and #3199 stopped new writes;
this normalises the existing rows so the column once again means
"no expiry" when unset.

Fixes: #3284

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>