Clearing a previously-set air timezone sent JSON null, which decodes to a
nil *string that UpdateMetadata treats as "skip this column", so the old
value remained. The dialog now sends "" (accepted by ValidateAirTimezone),
and UpdateMetadata maps air_timezone through NULLIF so an empty value
persists as SQL NULL (matching the nullable column) rather than "".
Addresses PR #21 review (P2).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
